Very interesting idea.
At first I thought the heart was required to move an alien. Would be interesting add this mechanic with a new organ. Maybe you can only control aliens that have a brain?
Would also be nice if the eyes did something. Perhaps the visuals of the game could be limited in some way if the alien you are controlling has no eyes. Maybe aliens that have eyes are too scared by something that they can see to move to certain squares, but blind aliens are able to move past scary things with no issue.
Limitless possibilities with this idea!