> One thing I wish it had was the ability to add an entirely new building.

If you want to add an entirely new building on the go, there is Vespucci:

https://f-droid.org/en/packages/de.blau.android/

But, honestly, adding a building on a phone sized screen, while quite possible, is much more easily done with either of the two desktop editors:

JOSM, Java based: https://josm.openstreetmap.de/

iD, Web based, this is the editor available from the "Edit" button from the OSM website: https://www.openstreetmap.org