Actually, I rather like that idea!
It would be cool and I don’t think it would be terribly overpowered for entangle to damage you if you attack. Especially since it looks like thorns. You try to wiggle out of the thorns and you’re going to get hurt.
And I like the idea of web doing damage when casting a spell. It actually adds some timing to your thought process.
Same with freezing doing damage when they try to get that mana (or perhaps only when they try to get a four of a kind of a frozen troops mana)
I really like that concept.