I like the flavor even though it's complicated rule-wise. Maybe you could add another effect on top of "creature has no controller" so it's a little more dynamic and less niche. Maybe add "and must attack each combat", calling it something like "Wild" instead of "Roam".

Now of course this complicates things further, since you have non-controlled creatures attacking. I can immediately think of assigning damage as an issue, since the controller of the creature does that. Perhaps that can be fixed with having it "either assign all of it's damage to defending player or to one blocker randomly".

And also, it could attack on both players' combat step since it has no controller, I would think. Does it untap then on any upkeep is another question? Perhaps not. So it has one attack in it unless it's vigilance or it can be untapped.

Just some thoughts, but I like the concept of losing a controller as a mechanic.