I feel like technical requirements/hardware limitations for something like Co-op are of a lot less concern and impediment than the design limitations and issues for implementing co-op, or even a decent AI partner, into a game like BotW. There's some pretty massive hurdles they'd have to overcome and solve for unless they were OK with some serious compromises to the experience in order to implement it. You can't just toss in a second player in that kind of game without outright breaking or at least seriously compromising a lot of things and/or needing to dramatically change your design approach and/or implement a lot of artificial elements to contain it.
To be clear I'm not suggesting that it wouldn't be a great challenge. However we know a lot of Monolith staff worked on BotW and likely will on its sequel.
Xenoblade Chronicles X is a giant open world game for the WiiU that includes a 4 player party and drop in online multiplayer.
The Switch is more powerful then the WiiU and the suggestion is one extra character. I'd be confident in the Zelda team quickly finding the solution