I'm not sure how to do it for an animal companion, but for summoning, they already have a 4E mechanic in place. The same way a wizard has to spend a minor action to sustain his 'fly' spell, a druid could have to use a standard action to sustain his summoned animal.Doug McCrae said:4e already balances 'lopsided' actions in the form of minions, elites and boss monsters. Though to some extent a boss is balanced by giving it more actions.
The issue with summoning (and animal companions and the leadership feat) is not so much balancing actions, it's one player getting more table time. Much, much more in the case of a 3e druid. A 3e druid with the leadership feat and his cohort is also a druid? My God, his turns are going to take half an hour each.
It would take some playtesting to find the proper execution of the idea, but that seems to me to be the way to go.