DA BIG BOI SUGGESTION (BY Talli#4951)

DJ type units
lv 1: Win 5 games with this unit with max top upgrades. +1% range
lv 2: Enhance 20 units at once -2% upgrade cost
lv 3: Win a game on insane enhancing no more than 7 units. -3% Upgrade cost
lv 4: Win a game on Insane with the bottom upgrade path +1% Range
lv 5: Win 20 Games on insane on Advanced+ maps +5% buff bonus (This applies to all dj abilities, for example 25% dmg boost would go to 30% with this.)

Miner type units:
Lv 1: Mine up 8k in under 2 rounds +2% cash yield
Lv 2: Mine up 20k in total -2% upgrade cost
Lv 3: Mine 5k in one round +3% Cash yield
Lv 4: Mine 100k in total +1 dmg
Lv 5: Mine 8k in ONE round. -3% upgrade cost.

Offensive towers, starting at scout level of strength and going up.
Lv 1: Kill 10 Bosses with scouts/snipers +2% Range
Lv 2: Kill 500 mobs with scouts/snipers. +1 Dmg
Lv 3: Win a game on hard or tougher solely using scouts/snipers. -3% Upgrade cost, +2% Fire rate
Lv 4: Kill 3k Mobs with scouts/snipers +3% firerate, +2% Range
Lv 5: Win 5 games only using scouts/snipers on medium or tougher maps on insane. +2 dmg, -2% fire rate.

(Solider/shotgunner requirements/upgrades.)
Lv 1: Kill 20 Bosses +3% Range
Lv 2: Kill 2k Mobs  +4% Firerate
Lv 3: Win 3 games on insane using these units +1 dmg
Lv 4: Kill 5k mobs -5% upgrade cost
Lv 5: Win 7 games on insane only using these units on meduim or tougher maps +2 dmg +3% fire rate

(Cyro/Pyro levels/boosts)
Lv 1: Afflict 5 mobs before one ends +2% Fire rate
Lv2: Afflict 1.5k mobs +2% range
Lv 3: Kill 2k mobs +1 dmg
Lv 4: Reach an endless wave of 75 with at least 2 of this unit in the team. +3% Fire rate, +2% Range
Lv 5: Afflict a total of 35 bosses -7% upgrade cost
(Fragger/Rocketeer, this one will be different. A slash will indict the differences for frag and rocketeer, since one is stronger than the other)
Lv 1: Kill 300/1k mobs with splash dmg  +3% Range
Lv 2: Inficlt 5k/20k in total with splash dmg +2% Range
Lv 3: Hit 10 mobs at once with splash +5% Fire rate
Lv 4: Kill 2/7k mobs in total -5% upgrade cost
Lv 5: Kill 10/30 bosses +1/+2dmg

(Minigunner, rail with have its own)
Lv 1: Win 5 games on insane maps with only minigunnners +2% Range
Lv 2: Kill 10k mobs +3% Fire rate
Lv 3: Kill 50 Bosses +2% Range, +2% Fire rate
Lv 4: Win 20 Games on insane on advanced+ maps -5% upgrade cost.
Lv 5: Inflict 100k dmg with minigunners +2 dmg
(Railgunner)
Lv1: Kill 200 mobs with a single shot +5% Fire rate
Lv 2: Win 5 insane games with only railgunners -5% Upgrade cost
Lv 3: Kill 50 Bosses +1 Dmg
Lv 4: Kill 5k mobs +10% Range
Lv 5: Kill 2k mobs with a single shot +2 dmg
(Engineer)
lv1: Build 1k drones +5% Fire rate
lv 2: Kill 2k mobs +5% range
Lv 3: Have 15 drones active at once  -5% build time
Lv 4: Inflict 6k dmg via splash -5% upgrade cost
Lv 5: Kill 10k mobs +2 dmg

(Madman)
Lv1: Kill 4k mobs +5% range
Lv 2: Deal 10k worth of splash dmg +5% explosion range
Lv 3: Capture 15 Pods using madman -5% upgrade cost
Lv 4: Kill 30 Bosses +1 dmg
Lv 5: Capture 50 pods using madman +10% Movement speed

(Berserker)
Lv 1: Inflict bleed on 2k Enemies +1 dmg
Lv 2: Kill 2k mobs +3% Range
Lv 3: Kill 750 foes with bleed damage 5% fire rate
Lv 4: Inflict 20k worth of bleed damage -5%
Lv 5: Inflict 100k worth of bleed damage +5% Bleed effect (Bleed does 5% more set%)

So some post writer comments, I grouped certian units together since their so similar in terms of strength or class. Like solider and shotgunner or fragger and rockeeteer. I spaced the more unique units on their own since they do their own job well. Minigunners for hordes, railgunners for single target nuking. I also capped the levels at 5, so that way its not to op yet its something to work towards for maxed players, yet new players don't have to stress about it. And as I said so before, this is my rough idea. Devs are more than welcome to mess with a few values they think aren't right. And uhh, forgive typos xD