The question is in the subject. I personally would say either animal crossing or Pokemon makes the most sense. What I'd like most to see, though, is a Mushroom Kingdom based MMO where you create a character who plays with thousands/millions of other characters to play through dungeons/raids/etc all based on various locations invented in Mario based games.

MMO market is too risky for Nintendo's situation right now... If you take a deep analysis on how the MMO market is going, you will notice that every game that releases with quality, ends turning on a fail or P2W game. MMO , imo, are way better suitable for PC only. We already have an upcoming ESO ( Elder Scrolls online, and will probably fail hard too, due to World of Warcraft's strong existence). So no, MMO is a bad idea for Nintendo. I prefer NEW Ip's and more games announced.

A new one. Your idea is in the right direction, Nintendo can make one worth playing. Campaign/Story mode should also be available in that MMO game IMO.

I just don't know how a Pokemon MMO would be executed. There's a lot of things that could go wrong. How would the gameplay be? Story? Servers? I feel they would have to bring in a ton of help for that venture.

I believe starting a little smaller might be more beneficial. ie) Animal Crossing. The gameplay wouldn't be as challenging and would be a great entry for Nintendo to get into online gaming. Just my two cents. It could also be a really big way to bring in casuals.

i hate mmo games so don't want them tainting their main IPs. Create a new one.

Damn, you gave me the "bolded, italicized, and underlined" treatment.

So you feel Nintendo could handle that entire venture by themselves? I'm not big on MMOs, so I do not know what they entail. So you feel it wouldn't have Nintendo's touch if touched by someone outside? Fair enough.

Of the choices you listed, Animal Crossing or Pokemon. Though I think Fire Emblem or the Xeno series could be awesome MMORPG's if done right.

I guess Pokemon would be the most successful cause Pokemon is giant followed by Animal Crossing cause that shit would rape every FB/Farmvillsk game on the planet and Xenoblade for the Monster Hunter/WoW people

Pokemon OFC I would sht bricks if a pokemon mmo came out that would be amazing.

Xeno would be a nice fit, but it's not worth the investment. Nor do I particularly want to see it happen. Pokemon would be a better fit.

Above all, though, I think Animal Crossing has the most to benefit from being an MMO. Even if it merely expands on the ideas that City Folk introduced.