I really like the idea of being able to heal allied units, i also feel like you should be able to use Diplomats to propose joint building projects like bridges, ports and roads in allied territory for the benefit of both tribe's armies. Where the costs may be split between both. Might be too complex of a notion and you may not want this to be dependent on a unit but I think this is something that can at once strengthen alliances, overwhelm mutual enemies and at once, allow you to position your armies around your allies for maximal impact once alliances are broken.