Tower Defense Ideas

Soon, i'll be working on my tower defense where u build on a cliff. Now before I do. I have no idea how to balance all this out.

Tower Race 1: Spellcaster Defense = Fire SC builder, Cold SC builder, Lightning SC builder, and Earth SC builder
Tower Race 2: Future Defense = Future Builder
Tower Race 3: Element Defenders = Fire builder, Cold builder, Lightning builder, Earth builder, and posion builder
Tower Race 4: Orc Defenders = Orc Builder
Tower Race 5: ???

Right now I thought of three tower races. Spellcaster Defense, Future Defense, and Element Defenders.

Spellcasters will be towers that will look like units, have mana, and cast spells. There spells do a lot of damage, but use up mana. Mana regen depends on the tower.

Future Defense will be towers that will look something that you probably haven't seen before. I don't know what I should do for this race.

Element Defenders = Fire builder has lowish-mid splash damage to ground and air.
Cold builder has lowish damage to ground and air, and slows down enemy units.
Lightning builder has high damage to ground and air, but slow attack spd.
Earth Element does high splash damage to ground, and high regular damage to air, but slow attack spd on both. Posion element does low-mid damage, slows enemy units, and stacks up with other posion towers.

Orc Defenders do midish-high damage, slow-normal attack speed, and few can slow.

The reason the Spellcasters and Element defenders have 4-5 builders is because theres more then one of that type. When you build a builder, its yours through-out the game and u can't buy another Spellcaster or Element Defender.

Could use as many ideas u guys have.

I need ideas for

Monster Health
Towers Damages
Tower Races
Future Defender
and any other ideas u got.