The answer is "probably not very much".
Despite this common perception, the reality is a lot thornier. It's impossible to predict alternative history, and I won't do so (it also doesn't square with sub rules). But I can describe the situation as it stood, and what followed the assassination, to help explain why I hold the opinion that the argument the process was derailed is overblown.
For one, let's consider what Rabin's positions were at the time of his assassination. Fortunately, we know pretty well what he was willing to do for the sake of peace, and it did not match up with what Palestinian leaders like Yasser Arafat were willing to accept. Rabin was assassinated on November 4, 1995. Rabin was on the record opposing a Palestinian state in 1994, and in 1995 he gave a speech to the Knesset just a month before his assassination. In the speech, he notably stated:
We view the permanent solution in the framework of State of Israel which will include most of the area of the Land of Israel as it was under the rule of the British Mandate, and alongside it a Palestinian entity which will be a home to most of the Palestinian residents living in the Gaza Strip and the West Bank.
We would like this to be an entity which is less than a state, and which will independently run the lives of the Palestinians under its authority. The borders of the State of Israel, during the permanent solution, will be beyond the lines which existed before the Six Day War. We will not return to the 4 June 1967 lines.
If this sounds familiar, it is with good reason; this position of a "state minus" is associated with what we might ordinarily call Netanyahu's position in the modern age. It seems unlikely it would have involved a Palestinian state in over 90% of the West Bank and all of Gaza, with a split Jerusalem. After all, Rabin's speech outlined a few key principles for a deal:
A united Jerusalem under Israeli control.
A security border in the Jordan Valley.
The inclusion of many major settlement blocs in Israel.
The establishment of blocs like Gush Katif (what was the bloc in Gaza, which was uprooted in 2005 by Israel's withdrawal) in the West Bank.
Why would this have been insufficient? Well, because Arafat rejected far, far better in 2000 and 2001. While I can't speak to 2008, I can mention what deals were on the table around 2000/2001, thanks to them being within the past 20 years. In 2000, Israel's offer is the subject of much debate, at least at Camp David. Some Israeli leaders on the left-wing have said it was justified for Palestinians to reject it, such as Shlomo Ben-Ami (then the Foreign Minister), but also pointed out that the rejection of offers later in 2000 and 2001 were inexcusable.
At Camp David, as best we can tell through the post-mortems, there was a dispute over what Israel's offer actually was. Dennis Ross, the lead American negotiator, included this map in his book to illustrate what he believed was the gap between how Palestinians characterized the offer and what the actual offer was. The rough details of the offer were for 91% of the West Bank, 1% worth in land swaps, and all of Gaza, as well as a capital in outlying Arab neighborhoods near Jerusalem. The deal at a minimum would have included around 86% of the West Bank, in Palestinian perceptions. Dennis Ross recounts pre-negotiations discussions with Palestinian officials who indicated 90-91% would be enough for a deal, but that obviously was not the case, as no deal resulted from this (or better) offers. He also recounts that an Egyptian advisor to Mubarak, then the leader of Egypt, said that the reason no deal came about was because the Palestinians realized they could ask for more.
None of this tracks with Rabin being able to seal a deal. Rabin's offers were significantly below even this Camp David negotiation. They were even further below later offers related to the Clinton Parameters and Taba negotiations, in December 2000 and January 2001.
The Clinton Parameters, for example, envisioned between 94-96% of the West Bank for a Palestinian state, land swaps equal to 1-3%, and a split of Jerusalem based on what neighborhoods were predominantly Arab or Jewish. The Jordan Valley border would be policed by Israel under international authority for 3 years, and Israel would have early warning stations there subject to review every 10 years.
Continued in part 2 reply to my own comment.