Micaiah - Uses the light tome Thani that shoots light bombs from above. Much like how it was effective against horseback and armored units in the original game, in Smash this would be translated to possessing shield breaking proprieties.
Black Knight - Using the sword Alondite, Black Knight shoots powerful projectiles at far away enemies, while delivering mighty blows to nearby foes.
Validar & Risen - The evil leader of the Grimleal would use the tome of Grima's Truth to launch projectiles, which upon hitting the floor would spawn Risen zombies. The zombies could be attacked by players. Validar's projectiles could also hit the players directly, though they wouldn't spawn zombies.
Caineghis - Appears on his human form shortly before transforming into his lion form, ramming mercilessly the opposing players.
Tiki, Nowi, Kurthnaga, Ena, or pretty much any Manakete or dragon laguz - Would act similar to Caineghis, except they would transform into dragons and breathe powerful blazing projectiles at foes.
Archer unit - Self-explanatory, I suppose. Would shoot arrows at opponents similarly to Link and Pit's special moves. Opponents hit in midair would receive more damage, as an allusion to archers' usefulness against aerial units. Could be a generic soldier or any of the many recruitable archers like Rolf, Shinon or Virion.
Dancer or heron - Using dances or galdr, these Assist Trophies would help the summoner by improving him/her and allies in a designated range. Improvements would include speed or attack power.