and a few more ideas :D
1)monsters that can carry multiple items.
2)monsters with hooks, so basically with a few seconds cooldown they bring items that are scattered around them to the path, but they can't carry multiple items just bring them closer.
3)monsters that are stronger then the normal ones, but they can't carry any items and if they are the only ones that made it to exit, it doesn't count as a win, basically monsters that are stronger than regular ones, but you can't have army made just from them.
4)maybe bring some interesting things into snake stage as well, multiple types of snakes and upgrades for them, for example snake that can make portal once a wave, it teleports them let's say 7 blocks away, or snake that can place buff area 2-3 times a wave, in this area monsters receive certain buffs. upgrades will obviously enhance their skills.
5)monsters that get stronger after each death and reverse, monsters that are stronger but get weaker after every death, eventually getting useless if you are careless.
6)funny idea but I think it's too specific and hard to make I guess, so basically it's unit that can move only in a certain way, by jumping, it jumps from one turning point to the other, while airborne he can't be damaged, he can jump only 5 blocks, if he can't detect turn within this 5 blocks range he just walks and is vulnerable , so you need to make a turn with your snake once in a while for this unit to be useful...it's kinda complicated so I think we can reduce this to just a unit that "jumps" instead of walking and is invincible while he's jumping.
7)I'm not talking about new types of defenders, because it's too obvious but what about traps? areas on the map where there is some kind of bonus(chest for example) but it is surrounded by traps, traps can be elemental(slowing, poisoning, stunning), or they can just damage your units.
8)maybe bundle of units? so when you buy one unit it's actually a bunch of them that are deployed simultaneously.
9)monsters that can interact with each other, so for example goblin and some kind of blob monster, when blob monster dies it leaves behind some substance, when goblin touches it, he becomes empowered. or goblin-lord, while he's alive every goblin-unit becomes stronger, lich-every undead unit becomes stronger and so on.
10)or a kind of unit that becomes stronger every time once of his kind dies, but only for this wave.
...yeah so many ideas, I hope some of them at least make it to the full game, good luck once again ;)