Every generation has had one of these and, assuming this is a new generation, I can't see it being any different. But it's never confined to a single humanoid Pokemon; there is NSFW fanart of pretty much every Pokemon out there if you look for it (well, I've never seen Dunsparce before...) and the best thing to do is just be careful when and where you search if you don't want to see it. Gardevoir isn't even the first Pokemon that springs to mind for me when I think of NSFW fanart, there is far, FAR worse stuff out there of Eevee and it's evolutions, and Charizard...and Lugia, for some weird reason. To name three I come across with alarming and irritating frequency when I'm after fanart.
The only reason Pokemon like Gardevoir get called out for things like this is because the art isn't really confined by image sites in the same way that others are, and because of its humanoid appearance, which means the artists don't have to get quite so creative anatomically speaking. Now, if you're asking if there will be more humanoid Pokemon, I'd say it's highly probable there will be at least one, but I don't expect it to be the only one to get swallowed up by Rule 34. I expect it to happen to every Pokemon, because it's the internet, so of course it will.
As for what it'll look like...well, I think that'd depend on where the region is, probably. I'd half expect a more humanoid Mega Bellossom if we're going to Hawaii for the region as rumour suggests to fill this "expy" role.
