If you think the mine drone is overpowered, try it. >.> Especially compared to other l4s, which blow it out of the water in terms of game-changingness. Medic's l4 is so good people rush for it, volt and mutant's l4 give them an extra life pretty much, dm's l4 can spell death very quickly, assault's l4 needs no words. And given how hard it is to do a mana-based special ops, the l4 really shouldn't be made to suck more anyway. Like I said, try it.
Are buildings being looked at? The last several games I've been in the pubbies have discovered the unbalancedness that is rushing for reavers. It makes a pvp based team fail totally (DM, volt, summoner) and short-range classes (archer, LM) a headache to play unless we respond with reavers of our own. The thing that I really find irritating about reavers (and spawns in general) is that they're always an option in a game, no matter the hero selection, but certain heroes deal with them better than others. They're also available to players a lot earlier than we are typically able to respond.
Something also feels off with the way assimilators work, in the sense that one person can supercharge his teammates with only 4-5 assimilators. Am I the only one who gets that vibe? I don't know...
Also, teleport receiver? Useless. Don't get me wrong, it's a good idea in theory (I would prefer the direction be reversed so that I could get to the temple quickly, but whatever). It has a build time of forever, though, and costs way, way too much money. The only times I've actually been able to build it are when our team's income becomes sillydiculous and destroying the temple is really just a formality.
Also useless are medics. (Not the hero, the EDIT: shield battery). The medics that spawn have ADD, and do not heal spawns in any meaningful fashion. Often when I use them (which is never in a serious manner), they're ignored entirely and end up as cannon fodder, although theoretically they could be a significant feed. In every case, though, they fail at what they were even intended to do.
Some building ideas:
-Antimagic Forcefield: Blocks enemy(maybe all?) spellcasting nearby.
-Time Dilator: Doubles the length of day and night. Would cause a time burner to explode if both were present on the map at the same time.
-Time Burner: Halves the length of day and night. Would cause a time dilator to explode if both were present on the map at the same time.
-Shield Generator: Makes nearby pylons invulnerable or makes the temple invulnerable if making pylons invul isn't possible.
-Aviary: Loads the hero into a shuttle which flys to either a destination building or lets them control where they head.
-Telescope: Grants night vision to heroes at the temple (ones who are invulnerable)
-Earthquake Machine: Halts ALL assimilator production.
-Doom Clock: After a warning and a countdown, destroys all pylons, assimilators, and the Doom Clock.
-Inn: Buildable healing place
-Temple Reviver: Makes the next hero who dies on your side not lose a life. The reviver is destroyed instead.
-Anti-Mana Tower: Alters mana counts of nearby heroes. May have huge range.
-Summoning Charm: After a warning and a countdown, summons ALL heroes to the charm and destroys it.
-Bomb: Kills anyone nearby. Maybe have a warning for this if its added, learning what the buildings did was trial and error, this would be a bad thing to figure out in that manner.
-Mana Fountain: Mmmmm... alters mana count for allies.
-Wall: A wall.
-Tech Building: Prereq for some upgrades?
Use these as inspiration for some real ideas.