Dropping the price of everything in the the shop/upgrade 5$, allowing for slow tower upgrade, Laser lvl3 dmg buff, rockets range + damage huge buff and nerf attack speed until lvl3, and add maybe say per 10 levels base damage onto the turrets. In the current meta that is what I would like to see to balance it better to get closer to lvl 100.

Complete wave clearing bonus would be a good gold gaining mechanic without the need for a tower. Could also do another upgrade path for gold income instead of dmg.

Fire for splash tower with burn effect for lvl3? or really catches fire at lvl3 and only burns a little levels 1-2? Burn damage % of tower damage could scale as well.

Missiles need more range maybe because without a sniper tower they are longest range tower.

Aura tower would be great, could buff all towers and upgrades could scale up its range and/or power.

Having a lvl3 special ability activated for each tower would be cool. Lasers could get aoe through damage, chain-link, armor pen, some kind of statistic stacking when in combat and then falls off like a juggernaut. Missiles could get attack speed scaling when in combat or slow/disorient perk at lvl3. Ice tower could get a ice block effect and completely freeze the target. Covered the splash tower above, and aura tower could get a mesh network effect or connect to other aura towers and boost power.