How about an upgrade where you can hack other robots to fight for you? two upgrade trees; More bots, higher level bots and you can only have so many of both; so you may max out numbers but you can't upgrade any in levels, so every upgrade takes away one (starting from the top) on the other tree so that it stays even (or not and you can upgrade both but that would be a little OP) and maybe you only have them for a certain period of time (bit I think having them on your side until the end but you have to destroy them at the end and if you hit them without killing them they become enemies again.) obviously you can take and add, these are just some ways to take it.
(I had originally posted this a month or so ago but accidentally put it under another game idea instead of making my own thread... I don't know how that happened)