Ok, found a Mansion, and at first glance it's a nice addition - I don't want to spoil it for you. (And I also haven't explored it completely.)
Just sharing a bit of the experience:
The cartographers seem to be pretty common. I thought they might be rare, but actually I saw more cartographers than good old librarians in new villages. (I'm not sure I really like this...)
So, I bought a Woodland Explorer Map from the first cartographer I found (I don't need the ocean map, I know 300000 ocean monuments...) and went for that Mansion. I know jeb said it might be far away if the player dot on the map is small. TBH I thought this might mean something like up to, say 3k blocks, which I think isn't far at all. But to my surprise and amazement, it wasn't that close - actually it was 22k blocks away! A journey with Ice Spikes, Mushroom Island, Mesa, and tons of Ocean Monuments, Desert Temples, Villages, Ravines, etc.
All in all, even if the mansion itself wouldn't be great, I think the explorer maps are a great idea to add another optional goal to the game.