If I had to pick a top 5, I would have to say:
1: Shantae
Shantae is one of the few characters I believe should be in smash, not just because of her history but because she's a fun bouncy character whom I believe would translate fairly well into smash bros. Also any reason to get Shantae remixes is a good one.
2: Rex and Pyra
I played through Xenoblade Chronicles 2 earlier this year, and absolutely loved every moment in it. I could see these two working well together, either having Rex as the main fighter with Pyra performing the specials. Though I'm unsure of exactly how they'd function, as it could be like XC2's main game or the Torna DLC (Haven't gotten round to it yet).
3: Leon S. Kennedy
This one is one I extremely doubt will ever get in Smash but one can hope. ('v')b Out of all the Resident Evil characters, I've always found myself liking Leon especially. I believe his jabs and tilts could consist of knife attacks and kicks, whereas specials could be different firearms. Smashes could be more kicks or shotgun shots. But I don't think he'd translate well into Smash.
4: Tails
I'm not a fan of the various Sonic characters out there, but out of them all Tails is probably my favourite. Him being the little technician he is, I could see fighting with a variety of tail swings and tools.
5: Isaac
I've not played the Golden Sun series since Dark Dawn on the DS, but regardless Isaac would be up in the top 5. With a good design and moveset potential, I could see him as maybe not the most aggresive fighter, but certainly use a few field (Move) and battle (Earthquake) skills alongside some swird swings for normal hits.