After watching TFA for the first time I got the impression that Luke went specifically to the first Jedi temple to learn something, perhaps even searching for the source of Snoke’s power.
Same.
The Emperor is the head of the Empire, which was the main threat of the original trilogy. Another separate threat would be introducing a new and separate antagonist. Lucas never did this. Snoke and the First Order was the primary antagonist, setting up Kylo to supplant him. Having an extra force to fight against that The First Order also has to fight against would have convoluted everything, be that threat another force or Knights of Ren.
Honestly, if in Episode IX something like the Yuuzhan Vong had shown up outta nowhere and the finale was about the First Order and Resistance teaming up for the greater good, I’d have been so down for that.