I don't think JJ had any specific ideas about why Luke was there beyond what was presented in TFA (That his temple was destroyed, he blamed himself, and ran away). Similarly, he didn't really have a solid plans for Rey's parents other than her being "a random girl who knows about the legends of Luke Skywalker". He talked about that in 2013-2014 when that's what got him into the project. The idea of a nobody/random person hearing legends about Luke and wondering who they were, etc..
IMO it just kind of furthers JJ's mystery box obsession. It kinda seems like Rey's flashback and stuff in TFA was just to start people going "what's going on?" when he didn't really have an answer for why those things happened. Same thing with Snoke, and same thing with Luke being on the island. I feel like a lot of exposition that should have been in the first movie (if it were going to be in the trilogy at all), was ignored in favor of the mystery box and it forced 8/9 into a catch 22.
The most obvious thing with 8 is that it needed to come up with why Luke would give up and peace out for 6 years, letting everyone die and all that jazz. I remember after TFA wondering why Luke would stay there when he knew Han was killed, the Resistance was getting ****ed up, etc - but also we all wanted to know what happened next, primarily because it seemed like it was cut-off mid scene. This resulted in RJ (or anyone that would have done 8) needing to pick up at that moment.
Something I had considered for a long while after TFA came out was that I wished there was a scene of them talking, etc. and maybe he starts training her at the end. But again, that wouldn't even be consistent with TFA itself. It would have basically been like "Hey, Luke basically quit and ran away left all this stuff behind.....but this random girl just showed up so he's gonna train her now within 5 min". JJ wrote Luke into a hole in general. Like, I would have loved for them to have some kind of talk at the end of TFA and the last shot to be them training during the sunset or something like that - but it wouldn't be consistent with the prior 2 hours of the movie. So 8 was kinda boned no matter what in that regard. It had to spend time on why Luke was there, and have development for him on the island to give us a reason why he would change his behavior after being on the island (whether it meant leaving the island or whatever).
I think it's possible Luke could have not died at the end of TLJ (which I would have preferred), but otherwise I don't see how his character being written completely differently in VIII and just being like "hey Rey you found me lets go train and kick ass" would be consistent with him in TFA at all.
And regarding Snoke and Rey's parents, after TLJ it seems like in 8/9 it'd just be exposition that takes up time and doesn't play a part in the story. It should have been part of TFA if anything where they were still setting things up. I really hope they don't do anything with it in 9 at this point.
I have some problems with TLJ, and some of them are just with that movie (Canto Bight could've been simpler and more fun that it was. Would've preferred the plot to just be breaking DJ out of jail and make him the codebreaker instead of the way it was. Scrap the fathier escape. Have them need to break in, break DJ out, steal the ship. The scenes on the ship about war profiteering were fine.) - but the majority of issues I see people blame TLJ for are the fault of TFA I think.
What they said. I'm sure it will turn out fine, although I really like it as is 
