One thing Shao Kahn and Shinnok in MK4 did was promise Reptile they'd resurrect his extinct race. So I wondered, why couldn't Raiden and/or the elder gods do the same, with the added difference that they might actually keep their promise, the key word being might, as Scorpion did get screwed in MKA. Whether they would do it or not, such an offer could win them a new ally, even if only temporarily.

I also wonder why Reptile doesn't think of joining the heroes. What makes Shao Kahn in particular more capable of resurrecting Reptile's race than the elder gods? Does he (Reptile) just believe that they would just flat out refuse anyway, so he sides with the one who said he'd grant his wish at some point? Maybe Shao Kahn told him the elder gods would refuse. Hell, he might have even told him the elder gods were the ones who wiped out his species. That could make a good explaination as to why Reptile sticks with the villains.

Of course, the explaination behind everything I mentioned likely has to do with NRS not being interested in developing Reptile's character in any way, nor do they care about treating him like a chump, and I get the feeling they don't want a "freak" like him among the heroes. Name me a good "freak" after all. Goro went back to being a villain (and a lackey) in MKD, Sheeva's rebellion against Shao Kahn was undone in the new timeline (thus leaving her...a lackey), and Khameleon is overwhelmingly underused plus she doesn't look at all reptilian (yet). There's also Nitara, but like Khameleon, she's underused, though other than the wings, she looks human, which does many aliens in the MK universe a favor. Just look at Kitana and Raiden, for instance.

But I'm talking about an in-universe explaination. That Reptile isn't smart enough to join the heroes wouldn't be surprising, given how NRS portrays him, but why wouldn't Raiden and the elder gods jump at the opportunity to recruit an additional warrior?