Here's my opinions on the concept, and I certainly like it, I think excess tokens should be carried over, (otherwise you once again weaken the drawback of undead), also, in the same vein you shouldn't "truly" be able to choose when to use tokens, but here's a decent trade off in my mind. Whenever a player plays a card with a keyword that has an associated token, the game checks to see if they have 100 tokens or more with that keyword, if they do, it activates, so, a player could "wait out" for a stronger card to use the side-effect with, or in the case of undead, wait for a cheaper undead card to cleanse their tokens and take less drawback, but, the drawback of waiting to take advantage of such would be not being able to play cards with that keyword until you're intending on using your tokens, seems quite balanced to me, a person would have to discard, or play cards without the keyword you're using to "choose" when to use/clear his tokens. (As to display, Maybe a box on the side where you could change between the token types, so you only need to watch the token values you care about with your deck. (As to seeing opponent tokens, your call.)