Hey Kad, welcome to Mazebert and the forum :-)
I'm glad that you're enjoying the game so far. Thanks for sharing your ideas here!
The first thing you mention is really that way by design. The idea is to give the player the freedom to combine item/potion cards anyway he likes to. This way the most creative strategies arise (like the one you mentioned with the pocket thief + herb witch + item buff). However, as you might not always get all the cards you thought would be great for your strategy in a game, you really need to improvise, and might even find a much better strategy while combining the cards you eventually got to something useful. I agree, in terms of balancing this really is a challenge! One thing going into this direction, is that you can find unique cards only once in a game, because otherwise their effects would be just too powerful.
Yes! The achievment system is something I have on my list. I haven't thought of combining achievments with benefits for the game, but that's definitely an interesting idea. Right now the only bonus you can get is by levelling up in the skilltree. In the next release, there will be the new foil card system, where you can win a special foil version of every tower (but its just an optical effect for the card...).
Yes yes I'd be a liar if I had said that I didn't enjoy the freedom of stacking accessories any way that I chose. I was merely making mention of the category system as a means of simplifying the balancing task to some degree. My brain stopped functioning after trying to balance a theoretical 100 cards with 100 items lol. The alternatives to such a system could go in a few directions. One method includes capping stats (which may already occur, I didn't check or notice it yet) and another method could include decreasing the spawn rate of items that could potentially be "tactically abused" as more of them spawn into the game world. No worries though, this post was an attempt toward a little think tank thingy majiggy ;)
Regarding the foil cards, once you obtain one, do you start every game with it or must you earn it each game? Unfortunately, because I am only level 20 my experience is limited and I have yet to survive a single game long enough to see even a level 30 tower. It'd be a shame to finally get one and then die the next wave without ever getting to utilize it. It'd be like intense gratification suddenly ripped away in a moment's time.
As soon as you earned the foil card you have it bound to your player profile.
The plan is to have a trophy box with all your foil cards in the main menu.
Also to have a new ladder where you can compete in terms of how many foil cards you already won :-D