I dont think they game is imbalanced that much at all. So any changes are more tweaks. Here are the tweaks I would not mind seeing:
1) Conversion rate for Erebus and Oak should be reduced. I am fine with a minion Erebus/Oak, I just have a problem with them being able to regenerate their army in 5 seconds.
2) All % items should affect base numbers, not numbers modified after item stacking. E.g., priest minions heal for a % of your base HP, fortification flag boosts by base, etc. The late game effect on HP that these things have really swings the game to one side's advantage.
3) Every 2 upgrades of the tower damage at the citidel should restore your defensive towers. That would provide some incentive to getting the upgrade. There is almost no incentive to get the tower damage upgrade.
4) Switch Sedna's healing wind with QoT's eat minions. That would give some reason to go QoT and some reason to pick up Yeti's. Sedna is just a beast right now.
5) Add a 15% slow to Erebus swarm that affects Erebus after a swarm. It's currently a get out of jail free card for Erebus and makes the guy damn neigh unkillable because he can always get out.
6) Push Reg's range increase ability back 3 levels. He should not be able to take down towers beginning at level 2.
7) Slightly increase Rook's movement speed.
8) Slightly increase Rook's mana cost for deploying advanced towers.
9) Add life steal components to some of the cheaper gloves.
10) Slightly reduce priest minion HP. It should be a viable strategy to tank down the healer pets and then turn on the owner, but it's neigh impossible now.
11) Increase summoning time for minions. Makes it more likely to be interrupted.
12) Slightly increase Sedna cast time for heal. It is really hard to interrupt.
13) Nerf Blood Vial to half it's current hits, but double it's hit regen.
15) Make armor mitigate abilities (like pounce, fireball, etc.)
16) Double cooldown time for lock scrolls.
17) Double cast time for applying lock scroll.