Hello.
First, I apologize for the bad english. I'm not exactly a native english speaker.
Most tower defense map I try tend to be quickly forgotten, but this one is both well done and with a still active forum, so I guess I can give a little feedback, it may help. Just remember that it's solo feedback ; I understand it's not your main focus, but still it can give good idea.
* some tower are both hard to understand, and somewhat boring. It's all those "twisted aoe shape tower", like spore, ice, oblivion, orbit, hydro, poison. WHen I got one of them, I tend to use them as a "general type aoe tower", unlike the well thought tower like for example disease, quark, gunpowder.
* I guess it can't be helped because of game balance, but some tower feel strange. The main offender for me is Ice in this regard. The ice tower make light damage, something that look strange, and it effect does not look like what ice could do. (unlike polar, that feel very icy for me ).
* Some other tower may be a little too much complicated ; for example vapor. It's already an unique tower because of it long range aoe aura, so adding the switch mechanism seem a little too much,because I feel it either waste uniqueness that could be used on other tower, or complicate fruitlessly the game. Tidal feel also a little too much complexe for my taste. Even if both wapor and tidal are a lot better that boring tower
* I love the fact that every piece of information are already in the game, but it also make understanding some mechanism harder. Phazer is a good example, the formula is good to know, but "does more damage the nearer the target is" is something I understand much more quickly than "does 123599/distance damage". I can't really see a good way to both have the formula and a good way to explain the ability, sadly.
* I does like that there is not a lot of slowing tower, but they may gain to slow in more different way IMHO. For example, 2 slowing tower and 2 stunning tower, or something like that.
* The Ronald round is very, very boring. I would like better upgraded trash from precedent round, while still not gaining any money and having boosted monster. Or at least ronald with elemental armor, and some of them with undead / quick / mechanical, that kind of thing. (but I play on hard and I don't see them all that often)
* I would like a button "random elemental" on the shop. Because full random change a lot of thing, and I just sometime want to have a little randomness to see what kind of tower I get.
* unlike tower, mob don't feel very element-specific. Having element-only property, or some other customization of elemental mob, could be fun.
It may feel like a lot, but still Element TD have a lot of good point, like being somewhat balanced, being well explained, having a good difficulty curve, being playable alone.
Suggestions
Tower :
* a hard hitting, slow firing tower with a chance of doubling the target speed for a little time. The idea is, it will accelerate some of the mob, but not all, giving a more spreaded wave that give more usefulness to tower with small range. For example, quark would love to have more time to fire upon mob.
* a tower that give a divine shield to a mob for 10 seconde. After thoses 10 secondes, the mob take a (very) big hit.
* a hard hitting tower that give a buff to the mob. The effect of the buff would be : "if this mob is not killed before the buff expiration, you loose one life". Basically, the best way I could create for an opposite to the life tower.
* I would like to see the return of a poison tower that give a long duration, life-sapping poison, and a tower with critical hit and a 1-1000 damage range. (but without any kind of composite attaque ) Both way to deal damage feel fun, and the poison one is also interesting to build around, since only one of thoses tower could be active on a given mob
* a tower that can divide a mob in two, each with half the life. Would be dangerous, but good with aoe tower.
* a tower that make a corpse explode for damage
* a tower that loose life with each of it hit, but it harder than most tower ; in effect, it would be the opposite of the gold tower, and would require some thought on when to sold them (and cause trouble in full random game, seem you won't even loose the 25% gold)
* a tower with devour : a long cooldown instant death (and with the cooldown depending of the hp of the unit).
Mob ability
* a mob that accelerate with time, and another one that decelerate. It would require more planification on tower placement.
* a mob that gain a set amount of life (like 1% or something) each time it's hit. Would make slow, hard hitting tower more useful.
* a mob that gain 3 stack of a speed buff per second, then loose one every second. Would make quick-hitting tower mroe useful.
* a mob that spawn another element monster when it die. Undead with a twist ;p.
I hope that you will find some useful idea in this post ;