Zoroark as a replacement for lucario seems plausible, given the record, but I can only imagine it as having a similar moveset to Wolf- claw attacks. Also, it would likely have an Illusion based counter attack (like lucario's double team).
Bowser jr would be... what, like a heavier hitting squirtle? I have no idea what kind of attacks he would have.
And as Wolf has been known to point out, Capcom is generally very generous in lending out characters for crossover battle scenarios and such. I would prefer BN megaman, but likely we'd see classic megaman with moves likely to be heavily based on the 2D fighters he's already in.
I would really like to see Isaac in, but in Brawl we saw them deliberately limit the number of sword fighters in the game. You can only do so much with sword styles before they all become the same stuff. But fortunately for Isaac, he also has psynergy and djinn and summons, fertile grounds for deriving lots of cool special attacks and final smash.
As far as third party goes, Nintendo may be more open to including more of them, but probably not if they have to pay the other companies some kind of royalty for using their character. Sonic and Snake were obviously last minute inclusions into Brawl, as evidenced by their late parts in SSE and their levels of broken-ness (snake in general, and sonic's final smash).