It's definitely going to happen eventually. Unlike other people in this thread, I have faith that where there's a will, there's a way. There were substantial roadblocks to Brawl modding every step of the way, but modders managed to conquer them and now we've found ways to reverse engineer and modify nearly every part of the game and even made headway with modding Melee. It took about two years after Brawl was released for mods to really catch on in a big way, and from there it just got bigger and bigger. If anything, the speed with which these things happen has accelerated. Remember how quickly the 3DS ROM was dumped online?
Mario Kart 8 has already been hacked, substantially so, and its only been out for a few months. Nintendo patched the exploit that allowed those modders to get into the system, and they're keeping their secrets to themselves for the time being, but this shows that the Wii U can be hacked, and more vulnerabilities will probably be found. No firmware is 100% hack proof. Whether or not it becomes the homebrew wasteland that the Wii was, we'll have to see. Personally I think we're likely to see 3DS mods first.
The fact that Sm4sh seems to use a modified version of the Brawl engine means that once the security of the Wii U itself is compromised, it probably won't be as difficult a road to get from simple texture hacks to things like PSA's as it was with Brawl, because we'd be working with most of the same file architecture as before. Furthermore, this thread and many identical threads prove that Sm4sh mods are something people want, so they'll start showing up eventually. But they're most likely years away. I wouldn't count against a total-conversion mod like Project M happening eventually, but it's a looooooooong way off if it does, and it won't be coming from the same team of modders, who have put enough work in as is.
Are there plans right now for any of this? I'm sure some hackers have entertained the idea, but like everyone's been saying, the game has been out for a week (at most) and DLC is on the way, so be a little patient! If you need mods, Brawl's still there, and there are people working on texture mods that make it look more like Sm4sh and even PSA's for some of the simpler newcomers. There's already a great one for Mega Man. Unfortunately the Brawl Vault has been having some serious server issues lately, but it should all be sorted out before too long.