If people desperately want player-controlled Guardians, why not a capturable Guardian Assembly Facility? It would work like the ASC and the AdvFac, only it would give you one Guardian type for every turret line you have unlocked up to max level. Human Guardians only function in supply (although you CAN take them out of supply, they can't use their weapons).
I guess that could potentially give you player versions of the Flak, Gravity, Heavy Beam, Laser, Lightning, Artillery (based on Missile Turrets), Spider, Tachyon, and Tractor guardians. There aren't any Guardian equivalents of Basic turrets or MLRS turrets; or of Counter-Dark-Matter, Counter-Missile, or Counter-Sniper.
That pretty much leaves a bunch of Guardians that would either be useless or overpowered in player hands, plus the Vampire Guardian, which I guess could follow on from the Basic turret chain.