Maybe JJ went out of his way to ignore them in 7, but I would disagree in saying that Lucasfilm or RJ in general went out of their way to ignore them.
In my opinion, I think that TLJ especially continues thematic elements introduced in the Prequels, like the flaws of the Jedi Order and the issue of what balance in the Force actually means. Kylo Ren begging Rey to join him in bringing “a new order to the galaxy” seems to clearly parallel Anakin offering the same with Padmé in ROTS.
Sure, there aren’t many surface-level references, but the OT doesn’t really have many either. And yes, it would’ve been impossible for the OT to reference the prequels visually when they were made before them, but if the OT doesn’t even have many retroactive prequel references, then why should we expect any at all even farther along in time during the ST?
I do agree though that it would’ve been nice for the Creature Shop to have thrown us a bone with at least a few more classic or prequel aliens in the new movies. I guess the idea is that they want an excuse to create their own aliens, plus they wanted to really push the idea that the galaxy’s demographics are incomprehensibly more diverse than we’ve previously assumed. I don’t think it would’ve hurt to have seen a one or two recognizable aliens from the other movies beyond returning alien characters, though.
Despite the other things I’ve said too, I do think it would be cool if they did throw out some more prequel connections though. Like for IX, I wouldn’t be surprised if they returned to a prequel planet, like Mustafar, Naboo or Tatooine. With the rough state the Resistance is in, I wouldn’t mind seeing outdated Clone Wars-era ships, and I think it wouldn’t feel too shoe-horned giving the situation. And yeah, it would be cool to see more prequel (and OT) aliens joining the fight.
So if we don’t get that stuff in IX, I could see that kind of stuff being included in a fan edit for that film. I think not seeing any prequel ships so far isn’t a big deal I wouldn’t really expect it.
Maybe you could change Hosnian Prime into Coruscant, though. You could take an image of Coruscant and put it on top of Hosnian Prime in the wide shot as the laser approaches and starts blowing up other planets/moons in that system. I would suggest taking out the other satellites since we don’t see other planets/moons near Coruscant in the Prequels (do we?). You would just need to make sure you cut out the one time it is referred to as “the Hosnian system” by Poe, though.
If Starkiller is Ilum, technically it would be a prequel reference since it was in the Clone Wars. You could try to make it clearer by adding more crystals jutting from the ground or hanging out of rock ceilings or cliffs, or Jedi ruins half buried in snow in certain shots.
You could possibly add OT/PT aliens to Maz’s Castle, Canto Bight, the Resistance or as an X-Wing pilot, but I think doing so would be challenging and more trouble than it’s worth, because you would be talking about needing to do passable CG work or practical shooting to integrate into the film.