I think the level 20 tower upgrades should become more umf to them, but not just in the idea of a passive, to change the gameplay around them.

First I think that the level 10 should remain like the passive buff that the tower gets and the level 20, instead of showing the choices you have: the ability and power to remain a secret until you choice originally the 2 from the 3 main upgrades. After selecting the main 2 upgrades it will reveal the choices you have.
Furthermore I came with some ideas:
The basic:
Attack speed -> Quad barrel: ads more attack speed but slightly reduces damage
Ricochet-> Twin Head: Get another head and has a mind of its own (same like missle targeting different targets)

The poison:
Projectile speed -> Hydra: ad 2 more heads, and slightly nerf de projectile speed
Initial damage -> Explosive Chems: Half's Poison duration, ads fire damage within a small area
Poison damage-> Toxicity: Poison damage now stacks, but its capped and stacks add a very small percentage

The tesla:
High Voltage -> Tesla Coil: the tower reduces its range by a good amount, but becomes a close road tower that deals continously damage
Chain Lightning-> Lucky Bolt: when the shot reaches its maxim length, it jumps backward from where it came dealing additional damage
Damage -> Zeus Bolt: the tower will shot lighting bolts even if enemies are in range

Sniper:
Damage : -> Current Ability 1.8.9: if enemies die within tiles they explode
Range: -> Power Shot: after locking on the enemy, charge for 5 sec a power shot
Piercing -> AP rounds: Ignores Shield from enemies, but reduces damage slightly

Freeze tower:
Experience-> Ads a button that when activated, it can share exp to the next towers, like a share modifier would do
Cold damage-> Tower gets a Barrel that shoots at enemies, similar to the Poison but it takes multiple hits to slow fully an enemy

Splash:
Piercing-> Current Ability in 1.8.9 when enemies die dispersed spikes
Attack speed-> Tornado: the more attack speed has the faster it spins
Damage -> Spikes: Reduces the attack speed and range but throws spikes that stick to the road

Missle:
Micro rockets-> Current ability in 1.8 9 micro missles lock on different enemies
Vertical launch-> Mini Nuke: The tower will start charging a missile, once fully charged the missle gets maximum speed hits the target and has big area damage, IT can Miss!
Anti-air missles-> Splitting rockets, upon the first enemy hit, explodes realising a another rocket but with reduced damage and area

Minigun:
Acceleration-> Current Ability 1.8.9 spawn turrets on the road with less stats
Damage -> Double Chain gun ands another barrel but no longer applies status effects, maybe a small chance to pierce 1 target
Rotation -> Leader: nearby towers will have the same targeting as miniguns, and have a chance to apply starts effects like Poison or fire

Crusher:
Experience-> Current Ability 1.8.9, dissorient enemies
Damage -> Anvil: the hook no longer grabs, becomes an anvil that extends to the maximum range dealing damage to all enemies in the way, having a chance to stun
Crushing amount-> Hunger : The more it chews the faster I will deal damage

Railgun:
Damage -> Current Abillity, increasing mining speed
Money generation-> Greed, for every X coins (500) increase damage, but also the amount of supplies needed
Self improvement-> Little Gus: summon a small gauss on the road, doesn't need to charge, and shoots on the first target, like a rifle man

So far that's all I thought of
The upgrades may seem overpowered.
I feel like level 20 doesn't offers a change in the tower
The only choice would be the ability, or power
Maybe would be good to have the ability as a 3rd option

Not to mention for new abilities maybe more enemies would be nice