I was working on à td not so long ago and had a nice feature i think
the builders are heroes with three stats:
Income - increases gold gained after each completed wave by 10%.
Tech - Each point increases the heroes passive skill by 1. Also unlocks new towers and upgrades.
Magic - increases heroes mana by 5. Also unlocks new spells for the hero.
The heroes have one passive skill (for example à damage bonus aura for your towers) which is upgraded by the tech stat. They also have four active spells (for example temporary gold boost from kills) which are unlocked at 5, 15, 30 and 50 magic points.
The magic skills have à moderate cooldown and if heroes start at 100 mana they can cost around 70-100 mana each. The heroes have no mana regen but regains all their mana after each wave. It adds more skill to the game to use your spells at the right time and also allows players to try different stat builds.
Heroes have à primary stat which increases by 1 every level and gets two skill points to put where they want. The heroes level up after each wave and gains no experience during waves.
Also it is nice with boss waves that has some tactics, examples:
1. Boss casts à dmg reduction spell which lasts à little longer than it takes for him to reach the point of interest, so you either need to slow him so the debuff fades out or some insane damage.
2. Boss activates à mana shield. Has large mana regen but not very big mana pool, so you need à strong burst in the beginning to break the shield so the towers behind can kill the boss (ideally cast à dmg boost spell on your towers for extra burst).
3. Boss summons à dummy add with so high defense it cant be killed so players has to manually order towers to attack the boss instead. Alternately the boss could summon adds far ahead so they dont have as far to run to the point of interest.
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Note: add quotes and special effects if you use bosses, make it clear its à boss
like weather effects, night/day change, Rain of fires or something at random places around the map