Ive ideas for 5 new towers:
1-Alloy
2-Samuray Asshole
3-Piconjo
4-Synj Asskicker
5-Strawberry Clock

you should put in picos friend( i forgot his name) and that demon from waterlolies.

I think it might be better if you extended it, as have what you have now then go repeat it. I know it would be hard but i think the long the better. Anyway it would also be more difficult since you would have to add more towers.

Sounds epic. To extend the length of the game, consider having an endless mode of sorts (if you haven't already, didn't read any other posts.) You could categorize each enemy into a tier and then run some code to randomly pick enemies as they spawn from that tier, of course have the tier increase be much slower in this mode to make the game seem like it's lasting longer. The final tier could be all enemies in the entire game being selected randomly, with 1.5 x the stats. Just an idea from a fellow game designer, take it or leave it, it's free :)