The most likely avenue for discovery on new Roman works is through archaeological excavation of papyri in dry climates like Egypt. Even discoveries like this are usually fragmentary, since entire, intact pieces of papyri are just not as likely to survive. The most well-known recent example of this was the "new Sappho" poem that came out a few years ago, found on papyrus fragment. The Dead Sea Scrolls are another famous example of this sort of discovery. These sorts of remains can come from trash deposits, or from outlying desert communities, so they are not necessarily representative of the bulk of what once existed.

Another potential avenue is new scientific techniques to recover lost materials. Sometimes, this allows researchers to see an original text after the writing material was re-used. There has also been a lot of effort in uncovering the Herculaneum scrolls without destroying them. These scrolls were charred and preserved in Herculaneum, a city destroyed at the same time as Pompeii. They can't be unrolled, but researchers have worked on elaborate forensic analysis to try and find out what they said. I believe they are mostly in Greek, so it's unlikely lost Cicero will be found there, but it is an example of the sort of research where new Roman materials would be found.

Even through all this type of research, it's still rare for new discoveries to be of previously lost texts. Usually, these sources give scholars a better context for existing materials, showing variations in how the transmission and copying of works happened over the centuries. But despite being rare, there is always the chance of a new breakthrough or discovery unearthing something that hasn't been read in centuries.