I'll only want a 5th generation of Pokemon on a few conditions....
1) Stop making Prevos/Evos of previously made Pokemon. I mean Steelix and Scizor were cool and Igglybuff, Cleffa, and Elekid were decent, but Rhyperior? Magmortar? Magnezone? That's just taking it too far. Not to mention Happiny, Mime Jr., Mantyke, Bonsly, etc. It's starting to get really old.
2) Make the Pokemon look like Pokemon. This new generation (and even the last generation) had Pokemon that didn't even look like Pokemon at all. The first two generations had Pokemon that looked cool and then after that, their looks began to taper off. Sure there are some cool looking Pokemon in D/P, but most of them look hideous!
3) Too many legendaries!!! Legendaries are supposed to be "legendary." D/P has, what 13 legendaries now? The amount of legendaries increases with each passing generation and it's really annoying. Each passing generation should have 6 legendaries at MOST! The legendary trio, the main Pokemon on the box cover, and an event-exclusive Pokemon is really all I need when it comes to legendaries.
4) No more tuitorials!! I already know how to catch Pokemon! I already know what a Pokemon Center does! I already know how to fight in trainer battles! I don't want them to hold my hand throughout the game because I already know how to do this stuff! If you're too lazy to read the manual in order to figure out how to play, then you shouldn't be playing the game at all.
5) No more useless extra features! When I use Cut, I don't need to see my Pokemon use the move. I just want to use Cut. I don't want anymore useless animations like this. I can stand moving Pokemon, but I don't need anything else. They could scrap the animations and use the excess space to add new, more useful features.
EDIT: 6) Pikachu wannabees need to seriously die too! We had Pichu, Plusle, Minum, and that electric squirrel Pach-something. We don't need anymore Pikachu wannabees. Especially because Pikcahu is one of the most overrated Pokemon out there.
Instead of making a new generation though, they can combine all the regions Johto, Kanto, Hoenn, Sinnoh, and the Orange Islands into one game. No more new Pogies. No more new legendaries. They can make everything the way it used to be.