I'm down for more synergy between Land and Ware.
I do think Land should stay somewhat distinctive from Ware as the two are different franchises with different ways of going at things, but I definitely wouldn't mind seeing elements or characters from Ware appear, like have Mona appear in the map as an explorer or what not.
While it shouldn't come too much at the cost of their unique identities and strengths, I think Wario as a franchise could heavily benefit from having more unity and synergy between its two franchises. Even if it’s just little things.
I don't really see any WarioWare characters fitting into Wario Land, especially if we're talking about the original games which had an edge to them, as opposed to Shake it!, and were pretty much solo treasure-hunts for Wario. Still, I could see them perhaps making Diamond City a sort of hub world if they were going to go that route. Have the WarioWare characters as NPCs that you can talk to, and do side missions for.
Buuuut I don't see them fitting in Wario's Castle in any way. That is known to be Wario's residence in the platformer games, and gave it a sense of continuity going back to the original Mario Land.
I think it would be strange, Wario land is by far on a different plane of existence, the wackiness is present in both ip's, with Nintendo's history of gimmicks, I would certainly hate to see what kind of monster they would spawn.
I think there shouldn't be too much overlap between Land and Ware, as that would blur the lines between these two completely different series, and as such would harm the uniqueness of both.
However, small things that go a little further than just references (Although these are cool too; I specically would like to see references to WarioWare in a future Land title) like WarioWare characters serving as NPCs or Diamond City having a role in the storyline would be just enough to keep the two series from getting too close, but things like these should still be approached with great care.
I think the best way to reference Ware is through the collectible treasures, so that it wouldn't be too obtrusive. Wario Land treasures already reference Mario games like the Yoshi Egg in WL2 and the Peach Sandals in Shake It.