Personally speaking, I feel like Tower Defense could use a lot of adjustments.
For starters,
Wither skeleton rushing is way too dominant of a strategy and majority of players have no idea how to defend this. I think skeletons need a price and experience buff. To send 12 level 4 skeletons, I feel like the pricing should be closer to 4000 than the current 2400, but I believe they should also provide more experience upon dying. Currently, defending 12 level 4 skeletons takes way more than 2400 coins to defend. Due to this, that is what leads me to believe a price adjustment should be made.
Archer Towers are currently extremely dominant when it comes to any defense. If you look at any base, it gets to a point that it is 70%+ archer towers, with a few mixed in ices, mages, quakes, and maybe sorcerers/zeuses. Although I believe it is okay to have one dominant tower, I don't believe in having one tower that is extremely viable versus any troop sent. Sorcerer towers are amazing versus ground troops, Zeus's are amazing versus anything, but it is single-target, and this makes it balanced. But then you get to archer towers and they stop everything *except invis spiders*. I personally find that wrong and it turns late-game defense into who can get more archer towers down quicker.
Creepers are useless. I'm not too sure what could be done to even make them slightly useful, but currently if you send any level creeper, they will just be lava potted. They have nothing special to their name and are not used unless you are facing a new player who doesn't realize how bad they are. Personally, I was thinking of maybe they could have the ability to attack towers, similarly to wither, but instead of destroying the tower, potentially stunning it. Unsure of how this idea would go, but I would be happy to just see creepers used in general. They need a major buff.
Silverfish are not a viable troop whatsoever. They are decent if you are facing new players, but that's about it. If you place down like 3 mage towers, you'll stomp them no problem. I think one way to make silverfish better is if endermites would be made fire resistant, therefore making them mini-blazes more or less.
Necromancer Tower is a great tower, if you go top path. Currently the bottom path is very under-used and could use adjustments. I personally love seeing new strategies and seeing unique ways of defending. Whenever I look at a base and see everyone placing the same towers, it takes away from the uniqueness of someone's gameplay, and currently, top path is the only one used. I feel like bottom path could become a lot more used if it had a special feature, for instance the snow placed by the snowmen acting as a slowness potion.
Witches are fine as is, but I thought it would be kind of cool if they could have the ability to use potions, just like in vanilla. For instance, if a witch is getting low, they could have the ability to drink instant health, or drink fire resistance upon taking fire damage. I feel like the fire resistance would make them a tad overpowered so it probably isn't a good idea though. Just a thought I figured I would bring up though.
Horde is a fun mode, but one of the nice things about, for instance wither, is being able to send a push with the wither and defend it. In Horde, I feel like you are incapable of using it to your advantage as once you send like 24 magma cubes, you hit an entity cap and it stops sending troops until your troops die. Currently, I feel like horde mode just feeds both teams money, and is not really a 'challenge' to defend. I think it would be interesting if there was a higher entity cap for it to be able to provide more for when a team wants to begin pushing the enemy.
Poison Tower is a nice tower, but you really only need one for the entire map. One poison tower top path is enough to last you all game, as it has a permanent effect. To add some variety to gameplay, I feel like a stronger option for bottom path, like longer duration, or a special effect could make it a viable option.
Giant Zombies are quite the laughing stock. Beating an enemy with Giants is a pretty embarrassing way to lose. Giants are extremely under-powered and are in desperate need of a buff. I'm not too sure what buff would be ideal, as I don't know how easy it would be to make it be too strong. But in the current way the game is, a few lava pots is all you need to knock out 12 level 1 giants. Yes, you ideally want to upgrade your troops, level 5 giants are not much better. If you have a maxed out defense, giants are more than likely never going to get close to the end.
Artillery Towers are nice all around, but I feel like bottom path could use a slight buff, especially early game. Currently, the tnt put on the field by bottom path is incapable of being set off by skeletons. This makes bottom path extremely useless early game, especially when being wither skele rushed. I feel like at least adding skeletons as a trigger for the tnt mines would make it a decent option.
Thanks for taking the time to read my comments. :)