In previous (around version 1.6) balance discussions, the balance between elements has been a topic of consideration. Around that time, Nature was weak, so Yggdrasil was added. That was good until people found that Yggdrasil could be helpful to any element.
Here are the relative strengths of the elements as I see it:
1 = weak, 5 = strong. The numbers are for strengths during each part of the game early, mid, late, bonus round and Vir.
Nature: 5, 5, 4, 3, 2
Metro: 2, 2, 3, 4, 5
Darkness: 4, 3, 4, 4, 4
Light: 3, 3, 2, 2, 1
(Edit: This is based off a discussion between me and Kami. There are likely things I overlooked or didn't consider, but I hope this makes a good starting place for a discussion on this topic.)
Nature has a weak late game, and Yggdrasil will soon be non-Nature, so Nature will be left with a weak late game. My proposal is to give the new Yggdrasil another trait: "Natural Affinity" that gives each nature tower a boost. I propose that boost is equal to the stats Yggdrasil has gained from potions.
With the new Yggdrasil and rings and this bonus, a nature tower could still have 3x potion effects. However, that is weaker in power than 2x potion effects with a boost from another element: Stonecutter or shadow adapt.
Also, while this would mean that every nature tower would get this boost, and the boost would be stronger than the boost from Abyss King or Stonecutter, the carry tower would be weaker. I think this will be fair, and if there is concern about it being too powerful, some of the effects could be reduced: luck, multicrit, or just reduce all potion effects from this bonus by 10-50%.
Metro is already very strong from stonecutter. The early game is a bit weak, but it can survive. There is no obligation to start metro, and if you try a metro-only build, it can work. Electric chair is getting a significant buff and should be a viable mid game carry. Maybe stonecutter should be nerfed, but it does require a large investment to make it work and most of the current metro carries are on the weaker side, blofeld excluded.
Darkness should be in a good place with the buff to shadow. Solara is already a good carry and Abyss King is a good boost tower for the element. It seems fair to me.
Knusperhexe is super strong. Once fully stacked, she gives a 10-14x damage boost to creeps within 3 range. This is similar power to stonecutter, but it stacks with stonecutter. Making her power only work with darkness towers could be interesting, but would change the overall game balance. This will be a topic of another post.
Light is the weakest element in my opinion. Lucifer has a strong blade and can be transformed into a neat dark Lucifer tower, but they aren't that powerful once the blade won't be able to be transferred by exploit.
New tower idea to give a damage buff to Light towers:
Idea for making King Arthur more powerful:
Making Lucifer more powerful could also help.
The other potential carries, Phoenix and Candle, are neat tower, but cannot realistically deal with boss type creeps without some help. I am making assumptions about Candle because I haven't tried using Candle as a carry since it seems designed and balanced as an anti-air tower.
Added links to other forum posts that relate to this topic.
Well as long as lucifer is transfomed hes again a non ligjt tower so his transformation doesnt count for light.
I guess he wont be played much after merge since his Blade couldnt be transferred which make it not that good anymore...
Since we have excalibur and Arthus Pendragon what about some Avalon related Towers / Items in Addition Oberon, Titania, Puk and so on.
Also Camelot relating Stuff? There is a lot Lore and Potential for some Additions :)
May this helps with your working :)
We dont have Merlin so far which could be an Powerfull Tower as well!