I'm not much of an expert on balanced gameplay, but I've always thought that having a unit that would generate extra resources so long as it remained alive on the battlefield would be an interesting twist.
Only problem I know that would happen with this is that such a unit would only be useful if the guy using it could successfully wall off a part of the field. Most of the time, unless the teams are just building up a crapload of units in a standoff, a wall-off means that you have enough units to win anyways.
Thought of it too to go back to the concept of an inf. unit for a tier 3 bank.
Maybe a laser unit with constant fire, huge damage and cost of 200 of every resource: 200 money, 200 population, 200 energy and 200 influence. Or a building that auto defends the base, and that building can be upgraded and more weapons are available to choose or purchase to defend the base. Or maybe an unit that flies between air and ground, and capable of attacking both.