Ok, I have two suggestions:
1. Avengers costumes for Heros! I know, I know, there's problems with copyrights and all, but it's just a suggestion. I was thinking of this kind of thing:
Thor/ Nick Fury = Malik Hammerfury,Hawkeye = Bolin Farslayer,Black Widow = Alleria Swiftwind, Hulk = Ignus, Captain America = Gerald Lightseeker, Iron Man/Loki = Magnus Spellbane.
2. A Level Four Artillery Tower: THE DWARVEN CANNON! Meh, it doesn't sound as good as "The Human Cannon." but it'll do.
Anyways, This upgrade costs 250 gold. Instead of shooting bombs, It shoots Dwarves! To shoot the cannon costs 35 gold (Maybe remove this and instead make it so that the dwarves disappear after some time?), and each Dwarf that is shot out deals 30- 50 (?) damage and after impact, turn into troops which deal damage and block enemies. They deal the same damage as reinforcements, and their upgrades are:
Sparklers: Gives Dwarves sparklers that upon contact, lighting an enemy on fire, making them run faster but they take damage over time.
1: Gives Sparklers.
Daredevils: Dwarves have a chance to deal more damage on impact, but at the cost of their HP.
1. +10 dmg; Dwarves take 50 DMG.
2. +15 DMG; Dwarves take 40 DMG
3. +30 DMG; Dwarves take 30 dmg.
And maybe a tie in with the Beer Tower idea? any ideas on that?