If a player is owned .5-1% of each attack is taken from players income/attack and added to owners bank. The total amount taxed would be listed on the attack screen with bfa and other such info. This sounds horrible at first. However, it would encourage players to buy active players over inactives. Additionally, the cost can easily be offset (and readily negated) by investing in ally's of their own. This would be a minor source of income for some and would help to offset the extreme cost that upgrades grow to be in this extremely inflated game. A minor flow of passive income would go a long way when talking about long term ownership. At the very least this may encourage owners to build a working dialogue with their allies. As opposed to the opposite, where allies are treated as, either, short term investments and quick flips, or long term stat bonuses.
It would make the big guys grow faster and throw off the equilibrium at first. BUT! Then allies become worth there cost in more than just ebs.