Y'know, call me crazy, but I've been thinking a lot about the NeoGAF leak, and that all points back to one big thing: The Rathalos, and its "appearance as a boss". I have two possible guesses.
A: The Rathalos is a STAGE boss for a Monster Hunter stage. While yes, not every single series in Smash Bros. that has a stage has a fighter to go with said stage, but I feel like it's different here. While most of those fighter-less stages (Nintendog, Hanenbow, etc) are mostly "dead" or at least really obscure,low priority IP's, Monster Hunter is far from being either of those. It's one of Capcom's best-selling franchises, and Monster Hunter World, in particular, is THE best-selling game in their HISTORY! So I just think it would make sense if Capcom didn't give Sakurai just a stage, a couple songs, and the Rathalos as an assist and a glorified hazard. Wouldn't you think there could plausibly be a fighter to go with that? Along with this,it's a third-party franchise, and I can't really picture a big third-party franchise getting just a stage and no fighter. Like, we didn't get JUST Green Hill Zone without Sonic, Suzaku Castle without Ryu, Dracula's Castle without the Belmonts, etc.
B: The "Spirits Mode" is indeed some kinda Subspace Emissary-esque Story mode, and the Rathalos is a boss THERE. In SSE, all of the bosses that were fought there (with the exception of the original bosses, like Galleom and, of course, Tabuu) were from series that were already represented with fighters. Petey Piranha(Mario), Porky(Mother), Rayquaza(Pokemon),and Ridley(Metroid). So if the Rathalos is a STORY boss, I can't see there being only Mario, Link, and Marth/whatever assortment of Nintendo characters fighting it WITHOUT a Hunter joining the fray to help them.
So yeah, maybe this is nothing more than the wishful thinking of a desperate fanboy,but I can't help being optimistic and HOPEFUL that the Rathalos, one way or another, will point to Monster Hunter joining the roster,and that will point to Terra.