Going to jump in here. I love Mother a lot. I was trying to do a run of EB before SSB4 (it's the only one I haven't gotten far in yet), but school is holding me back.
On the topic of other Mother characters, aside from Lucas's return... Well, it's sadly a small series, so even that would be great, but in a perfect world:
Jeff: He's already an assist trophy, and his gadgets would set him apart from the PSI boys easily.
Duster: His thief tools would be a blast to turn into a moveset. Too bad Lucas took Rope Snake.
Masked Man: While they could go the clone route since he uses PSI too, it'd be neat to see him specifically focus on lightning PSI, plus incorporate his buster, sword moves, and wings.
Porky: In his EB form, since that would be more manageable. His older form was already a boss, and he's a rare Mother character to be in 2 games, so why not? He could be another heavyweight character, though I guess
beat him to the punch of being our first mech user.
Giygas: Again, he was in 2 Mother games. His Mother 1 form would be the obvious choice, since how would you make his EB form playable? It'd also give us some more Mother representation.
New Age Retro Hippie: Because it'd be hilarious.
As a side note, what stages would everyone ideally love to see either in the Wii U version or future Smash games? I already got my two wishes (NPC and Magicant), but I'd love to see a stage based off the end of EB. Imagine Giygas as a boss.
Others I could see are Osohe Castle, Saturn Valley (it was in two games- if they don't just bring back Onett, it could be our Wii U stage), Moonside, and Threed (with zombies!).