Thanks for the opportunity to share some suggestions here! My ideas are about gameplay not so much about game balance. Some of them are generated from things I don't like so much others are just additions to what already exists:
1. Skip the tutorial in the campaign. You can create only one map (with a green flag design on the world map) with several levels (similar to campaign, heroic and iron but with other names of course, like Archers Tower Tutorial, Magic Tower Tutorial etc. and with different waves). This could be the only map with limited tower levels. In the Archers tutorial you are given access to all archer tower levels but others are limited, in the Magic Tower Tutorial you are learning to play with magic towers etc.
--- Reasons behind that: In every Kingdom Rush half of the campaign you are greatly limited choosing between two towers or all towers but with limited levels. I do not understand the structure of the game. I really like the IRON mode but not that limited campaign mode. At least when certain tower unlocks later in the game if you replay the older ones you could have access to the new options. Usually I play on Veteran difficulty in reverse order and when I reach backwards levels 10 or 9 the game is really boring. I do it only because of the achievements but I barely pay attention what is happening on the screen. And I have heard the same thing from several people on youtube when reviewing the game. From my point of view that is a design mistake if your customers thing something is boring in your game.
2. In the next installment of the game I will be very happy if you give us the possibility to replay the maps choosing races (this is why I named that topic KR: Legions) - Of course you will be creating a new race, whatever suits you, but after we finish the original campaign it will be cool if we can replay all the maps with the previous towers. The only reason why not do this would be the size of the game but in every year the internet connections are better and better and our smart phones/tablets come with more and more memory/hdd. Lets say you create a gnome race - after finishing all the maps a new option to replay the maps with the human-dwarf-or elven towers could exists. That would be a great possibility for you to finally rebalance them. This will give all of use like 4 times more hours to play, I have friends that feel that 15-18 maps, half of which you play with limited tower choice is just not worth it to buy the game.
3. I feel like all the 4th level towers need to have a special ability (like necromancer tower raising skeletons, or druids/archmages collecting charges when idle, or arcane archers/sorcerers lowering resistance/armor. In all previous game only few of the towers have had a specialization. Of course paid skills remain.
Thank you