With the exception of Steve, every character thus far has been from something that already had content, but really could have used more. Monster Hunter is currently the only series in the awkward position of having a boss and that's it. The rest of the bosses are either Smash-originals or their series are decently fleshed out. The stage isn't playable outside of the boss fight, and other than that we have Rathalos getting recycled for an AT, two music tracks, and a lone Rathalos spirit. And here sits the Monster Hunter series, finally hitting the mainstream in the west and only growing bigger, in addition to having been massive in Japan for a long time now. With a big new release on the way that is currently generating a ton of hype. An addition to Smash that would be E3-worthy, hands down (and could be even more timely if Rise gets DLC revealed at E3, which could very well happen). Really feels like if any series is next in line to fix its lack of content, it's Monster Hunter.
There's also the returning Mii costumes from Smash 4. Well there are only two left - the Lloyd one, and the MH armor duo. I think those costumes will return, giving all the more incentive to add The Hunter to Smash! I truly feel like The Hunter is the most likely character right now.
The only possible issue is that the Mii costume would make sense to return with Pyra & Mythra (both are JRPGs and both are duos) so we'll have to see what happens on that front.