I was reading about the way in which the Romans co-opted local gods into the own pantheon. For instance with Mars Mullo and Mars Camulus in Gaul, who were originally major indigenous gods that became worshiped across the Empire as manifestations of the Roman God Mars.
I assume there were attempts to do the same with Jesus, and I know the Romans originally tried to convince the Jews that they were worshiping a version of Jupiter, but I’ve never seen an example of where it succeeded even on a local level.
Can anyone tell me more?
The example that comes to mind is the case of the Roman Emperor Alexander Severus. Alexander Severus did not attempt this on a broad scale, but in certain limited cases.
In the Historia Augusta, Alexander Severus is said to have worshipped in a private chapel, or sanctuary, a number of Gods, historical figures, and Demi-Gods, similar in the manner to Marcus Aurelius and Alexander the Great. On top of this, when Alexander Severus had returned to Antioch, his mother, Julia Avita Mamaea, personally summoned the Christian Theologian Origen to tutor the young Emperor in Christian Philosophy, as well as Neoplatonism.
His manner of living was as follows: First of all, if it were permissible, that is to say, if he had not lain with his wife, in the early morning hours he would worship in the sanctuary of his Lares, in which he kept statues of the deified emperors — of whom, however, only the best had been selected — and also of certain holy souls, among them Apollonius,116 and, according to a contemporary writer, Christ, Abraham, Orpheus, and others of this same character and, besides, the portraits of his ancestors.117 3 If this act of worship were not possible, he would ride about, or fish, or walk, or hunt, according to the character of the place in which he was. 4 Next, if the hour permitted, he would give earnest attention to p237 public business, for all matters both military and civil, were, as I have said previously,118 worked over by his friends — who were, however, upright and faithful and never open to bribes — and when they had been thus worked over they were given his endorsement, except when it pleased him to make some alteration.
Furthermore, according to the Historia Augusta, Alexander Severus had also considered formally building Temples to Christ, which was also apparently considered by Hadrian.
Every seven days, when he was in the city, he went up to the Capitolium, and he visited the other temples frequently. 6 He also wished to build a temple to Christ and give him a place among the gods177 — a measure, which, they say, was also considered by Hadrian. For Hadrian ordered a temple without an image to be built in every city, and because these temples, built by him with this intention, so they say, are dedicated to no particular deity, they are called today merely Hadrian's temples.178 7 Alexander, however, was prevented from carrying out this purpose, because those who examined the sacred victims ascertained that if he did, all men would become Christians and the other temples would of necessity be abandoned.
The Life of Severus Alexander, Historia Augusta (43:5-7)
However, it remains difficult to determine whether these were part of an organized campaign of syncretism, or if they were one individual emperor's idosyncratic religious ideas.
This is one of my favorite things to think about, so I'll give it a shot. Disclaimer that I'm firmly in the early Empire/New Testament period, so hopefully some late antiquarians can weigh in on later developments.
By the third century there was never, to my knowledge, an attempt to bring the worship of Jesus up to a civic level alongside traditional deities--meaning there weren't major temples or festivals to Christ as part of any civic calendar (although, as I said, late antiquity isn't where I live, and it's possible that if you apply some of the questions I'm about to talk about to later events, you could start to see them differently than they're traditionally considered). However, it's possible and perhaps likely that there was a long-ish period where many people were incorporating Jesus into their normal ideas of piety without doing anything that we would consider "conversion."
The first thing is that Greco-Roman society did not recognize exclusivity of cult as a value or a necessity. Meaning, you didn't choose which god(s) to worship at the expense of others. Worship of the gods was built into the civic, social, and cultural worldview and was entirely about action--for example, making the correct sacrifices at the correct times--and ethnicity (which is to say, ancestral tradition). Nobody, except some philosophers, was really asking whether you believed in it or believed the correct things about it. They were aware that Judeans required exclusivity, but that was one of the things that made Judeans suspect in the Roman empire, even if their practices were legally tolerated.
And that brings us to the second thing, which is that the answer to your question heavily depends on who you're reading. Unfortunately, the bulk of the literature that's been preserved from the ancient world was written by and for the elite. If you spend too much time with Cicero you might think that people were skeptical of the gods all over the place, but Cicero doesn't represent more than half a percent of Roman society. Same with the New Testament--those aren't "elite" texts in the sense that they were written by rich and highly educated people, but they are in the sense that they were considered authoritative by Christian leaders from pretty early on. The NT isn't unanimous about hardly anything, but perspectives that were too alternative didn't survive, or only chanced to in things like the Nag Hammadi collection.
So in some ways your question is impossible to answer. The Jesus people whose opinions survived are the people who were most invested in exclusivity of cult. It's a minor but growing contention in the field that especially in the first couple of centuries there were a LOT of people who didn't have that investment--after a certain and pretty early point most people in the Jesus tradition were non-Jews, and nothing in their cultural background required that they pick and choose their gods. But the evidence we have for those people is very easy to read through a more orthodox lens. The Pauline literature, and especially 1 Corinthians, gives us peeks at this dynamic, but you have to squint past Paul himself to see it. There's a recent paper in Journal of Early Christian History about the section in 1 Corinthians where he talked about eating meat that was sacrificed at polytheist temples that speculates on this topic. "Sacrificed Meat in Corinth and Jesus Worship as a Cult among Cults," by Margaret Froelich, if you have library access.
From the other direction, the non-Christians/non-Jews whose opinions about Jesus survived are almost exclusively in the "Christianity is a dangerous superstition" camp. Pliny the Younger is one of the earliest (letter 10.19, which is short and fully google-able, unlike the academic literature), and he never questions that Christians should be punished, just whether he's doing it the right way. "Depraved, excessive superstition" is one of the ways he describes Christianity. He tells us of people he arrested who happily denounced Christ and worshipped the emperor and the Roman gods, and what we're supposed to see there is people who either left the group years before or were falsely accused in the first place. But the option that's usually not considered, by Pliny or anyone else, is that there may have been at least a few who didn't deeply feel a contradiction between worshipping Christ and worshipping the emperor, especially when torture and death were at stake. Their leadership was probably, like Paul, warning them about it over and over, but one of the major principles of academic biblical study is that you don't have to warn people about things they're not doing.
We also have a lot of evidence of people syncretizing Jesus with other gods. Dennis MacDonald and Courtney Friesen in particular, but also others, have gone on record saying that the Gospel of John takes a whole lot of inspiration from the Dionysus cults. Guess who was miraculously producing wine a long time before it was cool. Here's Pausanias (6.26.1-2, trans. Jones and Ormerod, from the Perseus Project):
The place where they hold the festival they name the Thyia is about eight stades from the city. Three pots are brought into the building by the priests and set down empty in the presence of the citizens and of any strangers who may chance to be in the country. The doors of the building are sealed by the priests themselves and by any others who may be so inclined. On the morrow they are allowed to examine the seals, and on going into the building they find the pots filled with wine.
Art depicting Jesus with the iconography of Hermes, Helios, or Apollo is also common. Here is a mosaic of Christ that uses the iconography of Sol Invictus. In fact, sometimes the only way to determine that it's Jesus being represented is the fact that the art was found in a church or a Christian tomb or something, but that in itself involves a lot of assumptions. By the time you get to late antiquity Jesus is so popular and embedded that sometimes the direction of syncretization isn't entirely clear--Julian's Aesclepius is probably the result of a whole lot of back and forth between Aesclepius and Jesus over the centuries. These kinds of things are usually labeled "competition" in the modern literature, but it's also possible to recognize that Greco-Roman culture had a particular vocabulary for talking about and depicting gods, and it would be natural for people who understand Jesus as a god to bring him into that vocabulary without necessarily intending to displace the traditional gods (Julian is not an example of that, manifestly, but other instances could be).
Finally, there are a lot of magical texts, especially in the 3rd century and later, that use Christian and Jewish language, names, etc., side-by-side with traditional Greco-Roman personalities and ideas. The usual way I see this talked about in the secondary literature is "pagans" who recognize the power of the Jewish god and Jesus for magical purposes, but if you warp that lens just a tiny bit you get people who have incorporated the worship of Jesus and the Jewish god without exclusivity, just like they incorporated the worship of Isis, Mithras, and the Syrian mother in the centuries before.