Team Bonus for Safehouses
The problem: Currently, because of the founder and leader bonuses, there's a disincentive to contribute to other players' safehouses; you get more cards from dropping survivors in a lvl 1 safehouse that you own than a lvl 20+ safehouse that another player owns. This renders safehouse leaderboards kinda pointless, since contributing survivors is wasteful for everybody except the founder. It's particularly a shame when it comes to safehouses owned by teammates, which would otherwise be a good opportunity for cooperative gameplay.
A simple tweak that could improve this: Make the founder bonus apply to all members of the founder's team. That would encourage team members to help each other maintain high-level safehouses and provide a reason to compete for the leader position, while retaining an incentive for each player to build up their own safehouse.
Honestly I'd like even more fleshed-out reasons to interact with safehouses, both my teammates' and those I stumble across... but this tweak would be a start. At least it would make the multiplayer element of safehouses not universally pointless.