One clear idea that TLJ leaves IX to pick up with: rebuilding the Resistance in the wake of a resurgent First Order, using Luke's final act as a rallying cry to do so, and resolving the war between the two. But in this movie I actually have no idea what is going on with the First Order (have they just been reduced to Kylo's Star Destroyer? seems that way but it's not clear) because the movie sidesteps them in favor of creating the Final Order as the climactic final battle for the story. Kylo Ren becoming the new leader of the First Order as we've been talking about is also a clear area of story potential for IX to dig into. You might disagree with that as the right characterization for Kylo, but it's not "nowhere for the third film" to go, it was definitely somewhere to go, it's just not an idea JJ pursues for even a second of screentime.
I agree that making Palpatine the overarching villain of the series is not a bad idea at all. I think it could've worked. My issue with him here is that in the end the movie never really justifies Palpatine's return and his inclusion creates a lot of other story problems.
In the movie, Poe says that no one answered the call from Episode 8 and that's why he didn't believe anyone was going to come for the final fight in 9. It was weird how JJ again decided to continue to cripple the good guys, I guess to set up this final scene of everyone coming to their aid.