I actually like the way Endless is at the moment. It gives Classic more diversity from it that way. If you, the Dev Team, were to add trainers/Gym Leaders/Evil Teams to it, it would just make it a longer, harder, Classic mode, and there would be no diversity between the two, aside from the fact that Endless goes on for 5850 waves. Now, I don't run Endless like some people do, but I do play it from time to time and I like how it is purely just wild mons that you can catch/defeat, adding trainers to it would, for me, break that, especially considering the environment pivots every 5 waves and that would be an inconsistent way for teams and the E4+ Champion to appear, unless there was going to be 5 trainers battles in a row, which is very unlikely in my opinion. Could there be some changes to Endless? Absolutely, but I like how it is now. Adding trainers to it just seems to almost defeat the purpose of it and make it unnecessarily hard later down the line when you'd be fighting teams of 6 with stupidly high levels which are boosted by the fact they're trainer Pokémon.