I want a Majora's Mask-like sequel. Breath of the Wild was, just like Ocarina of Time, a great game to show off the new possibilities (the size and openness of the world), it was a very exciting game to play because of how different and innovative it was. The sequel won't have the same level of excitement if it's just BotW with another world, so a bigger focus on story and atmosphere would help it stand out (BotW's storytelling was great, so I hope they keep that, but the plot was a bit on the light side).
Other than that, I don't think any of BotW's mechanics can really be improved, but I do hope they bring enough change and variety: new enemies, animals, plants, objects, weapons, clothes... and also fresh ideas for shrines, koroks, dungeons...