Looking to alternate history I am always interested in stories, which lead to a better and more stable Rome.
Gracchus:
I doubt a succesful land reform would change that much regarding the main issues of the roman republic. Perhaps if it would lead to a situation, where the senate becomes responsible for the pension scheme of the legionairs and more importantly sees the need to take care of it with passion. I doubt, this works with the aristocrats of these times.
So the challenge is, to analyze, what the issues of the roman republic have been and check, how Grachus could change them.
Caesar:
Is an old and probably ill man. His attempts to reform the republic were much less substantial than Octavians ones. So I doubt again history would become that much different.
However, if Caesar defeat the parthians and manages to implement provinces in at least Mesopotamia and Media in a way, that they are longerm controllable from Rome in a stable manner, this would change the world dramatically. I just have no clue, how this should work. It is actually a mission impossible.
I asume, that his heir would be again Octavian, but now coming from a much better position without a civil war.
Lucius Verus:
I doubt he would change that much. The 3rd century crisis will happen, no matter what. The structural changes, which are already running are way too strong. One guy can't butterfly such strong changes away.
Well, you could say, that with Verus, the success-model of adoption and co-emperors could last forever. But first you have to answer the question, why the adoption model failed and why it should not fail with or after Verus. Also later it never worked again. At least not for long. Even the tetrarchy with 4 emperors did not work. And again the social, economical and military changes are strong these days. Commodus was surely not the main issue. If Commodus was a problem, why Severus Alexander was not? The problem is far more sophisticated.
Of course you could always write interesting stories, which proof that these guys will change nothing and after a while we will end up with the same well known history. But what's the challenge with such an approach?