I feel like making gralats unable to spawn on bushes would fix the problem. For gralats that are already on the bushes, you could probably have them be automatically added to the player's account, but this would probably make a lot of people mad.

Even if the system is changed to a rating system I don't think it will change much. The leaderboard is really centered around how often the house is seen. Players that advertise their houses more will still be ranked in the top.