uh, first you need friends (or strangers), can't really work without them
then you need a bit of fantasy. Engine comes as third. I've had my home in NWN persistent worlds. Even when engine could be considered a bit limited, a lot of things was done at that home
Much more then any engine could do unless full motion scanner and VR comes as common (may Seven bless OR).
Anyway I like D2 tower. A bit simplistic but I liked the fight for it. Would be great to have that in D:OS. Could be even more interesting if f.e. broken things during the fight would mean they remain broken afterwards and needed more resources to repair them. Room rearrangement ala Runescape
(a bit UO style, but that game have so many fresh ideas) would be great either. With biiig garden and zoo
Or at least with ability to place trophies from big monsters (I wanna row of wolves heads)
And/or it was good to have home in NWN2 (singleplayer). One chapter was about improving your home, buying new rooms and buildings (it was castle with lands) and traveling to gain allies for upcoming battle. Then you command (place) groups of units you've convinced on your side (to first wall, outside, inside left, etc.) and then battle began. That was good.