Strafe Solo TD

I'm looking for good ideas for my solo TD called "Strafe Solo TD".

Some information about the TD:
The TD is a bit complicated. You have several tower powerups, such as elemental level, exorcism level, etc.. With each tower you build or upgrade, you get more of one kind and your towers will grow stronger. Im planning to implement more then 200 levels to survive. The main key in this td, isnt mazing. It's just building some towers on a higher plateau in the middle of a lower earthern line. This TD is mainly focused on killing incoming waves of elements and demons.
What I have already:
- Elemental and Exorcism Class
- 20 Enemy Waves
- A Tutorial for an introduction to the TD
- 10 Towers

Needed:
What I need is some1 with good ideas for gameplay increasing features., cauz it's a solo td and it would be boring without funny stuff and good gameplay. Since it's my first map, maybe you could give me some help with the trigger (or not^^).

Of course some Game Modes would be nice.
I am planning Race, Normal, Quest (where some kinds of towers need to get more kills or you have to reach a certain level with a certain power), Ultimate (All units have speed and higher armour. It's kind of an difficulty, but in this TD the difficulty only influences HP). Some great ideas would be nice here, too!

Also i need a few tower ideas! With that i also need special tower classes (like elemental level or exorcism level)!

If you can be so kind and just post your ideas here, I'll have to thank you.

If you are asking for ideas, shouldnt you be posting in the Ideas section?
But that aside, why not(will only work in race mode though I suppose it will not make a difference if it happens in other modes) have some levels where every so often for 1 level you get the guy above (or on the side) of you's level so player 1 gets player 2's creeps or something like this. This will make it interesting in race mode because then everybody needs to keep up with each other in order to survive.

its solo^^

New Stuff:
- I now made a system wich tells you how much kills and damage one tower type made (like you have a arrow tower and you click on it it then a window pops up with a kill/damage-count)
- Combination of some Game Modes
It's now possible to choose between a combination of game modes. For example, you have chosen the Ultimate game mode at first, then you can combine it with quest and/or race.

Need Help with:
What effects the gameplay:
I'm trying to make it even better in order to tell the player how much one tower killed. It will be optional. The hard part is that I need one infinite variable wich declares the tower number (the order in wich the towers were built).

I'll guess I have to balance the game before it gets released. I'm looking for some testers! (Some time left but It'd be better if you right now!)

What effects the towers:
- Some ideas for tower classes !HELP!
- Some ideas for tower spells !HELP!
- An interest-system (something like in Element TD) !HELP!
- Need a upgrade-system with tower-upgrades

What effects the ressources:
- Need more (besides lumber and gold, special points you can use to build more advanced towers)

Planned stuff:
- 200 level max
- More game modes
- Involve your ideas into the game if you come up with good ones!!

Thank you for reading this and special thanks for every comment!!

what would be n1 will also be a own user interface! Need someone to do this for me^^