Personally, I always thought having Tom Nook as Villager's echo made the most sense since he sells Villager all the equipment he/she uses. I would want Isabelle as a fighter that used items that Villager either doesn't use like the hammer and the fishing rod or uses very little like the gardening can. She could also do normal secretary stuff like falling with papers in her hand similar to Villager's dash attack or splash coffee like a mad man. Just my two cents, though.
Though I wouldn't have an issue with Mr. Resetti being in as well.
On another note, he coincidentally has the ability to bring unique stuff to Smash. How fitting.
Likewise, Mr. Resetti couldn't do what Villager does. Isabelle clearly can, but I feel she wouldn't. Well, not as a simplistic Echo like Lucina. Villager is clearly designed to be a bit darker than the series shows, with a little bit of happy-go-lucky. Isabelle is all about being happy-go-lucky. If you make her items toy-like, and keeps her always a happy/fun expression, she would feel more accurate. I'd like a different moveset, but at least this feels like it wouldn't clash too hard with her personality of "no way she would deliberately hurt someone". If she thinks it's all fun and games? Perfect! But an accidental fighter doesn't make sense. She'll figure out she's hurting someone and wouldn't like it. She's very very smart. Not that I don't think she could lack a dark side, but still, her own personality in Smash establishes her as someone who wouldn't want to hurt others. She shies away from fighting. I think Sakurai will work her personality in to keep this trait.
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Back to Isaac, I'm unsure how much I want his weapons to show up. I mean, his signature weapon is a sword, so at least some usage. This isn't like with Ganondorf who didn't officially use a sword till WW.