That could be interesting, especially when drones get beefed up a little bit. Right now the only real choice would be "do I bring my servo back a little sooner or save for a servo weapon/armor upgrade?" When drones are up the choices branch out to building more drones, upgrading them, etc...
One thing that could be interesting:
Have the spawn timers for servos start the moment the servo dies, for all servos at once. Yet have the timers on a 3 or 4 minute timer for each. That way its not only "do I upgrade/build or get the servo(s) back sooner?" It's "which servo(s) do I bring back quicker and how many?"
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Or here's a crazy idea:
The more bounty kills you get the quicker it speeds up the servo timer? That could give drones a greater value if they're beefed up a bit. I dunno that seems a lil too arcade like.
Though, that could actually be kind of cool for a pvp scenario game type... Servos only respawn based on victory points held and bounty kills. Victory Points (seperate from bloom refineries) generate points that are only used to respawn servos, and bounty kills add to those points. Have like 3 Victory Points on the map total.