As great as it would be, I don't think there's ever going to be an increased difficulty mode. It just doesn't seem like the sort of thing Pokemon would really do. It markets mainly toward kids but makes itself open to all; it's a casual game anybody can play. Adding levels of difficulty, even if there is the good old easy mode option, makes it a little less casual and a little more... hardcore, i guess? Which goes against the spirit of what Pokemon is all about. But that's just me speculating, and I could be way off. After all they did include the "Round 2" increased difficulty mode for Pokemon Stadium, which becomes accessible after completing the standard mode.
But, if it does happen, it'll probably just be like... higher levelled opponents, with more Pokemon or Pokemon at higher stages of evolution, and more type coverage with their movesets. Maybe a higher-levelled evolution system so your Pokemon evolve later. And possibly better IVs for your opponents. It certainly wouldn't touch on EVs, though. EV training is a means of manipulating the game mechanics to choke out the best Pokemon you can, devised by fans and done by fans. A harder difficulty level might include Pokemon with a bit more specialized stat layout, but there's no way it'd be like legit competitive battling in-game. ... And there definitely won't be a Nuzlocke mode. I highly doubt the people who make the games even know what a Nuzlocke is, and at any rate Pokemon dying doesn't exactly work in a game intended for children.
I'm pretty content with Pokemon as it is. You can already battle competitively or run Nuzlockes on your own. If they did make a game with increased difficulty options I would definitely play it and enjoy it, but I can't see it happening and I don't really mind.