Yes, I think you might be right about not being too harsh. It's pointless to be able to move around if you can't actually do anything!
After some more thought, I'm leaning towards:
- When you hit zero HP, flip your board to the Wounded side.
- Hit Points when Wounded are half your normal amount (i.e. surge value) and you have Disadvantage on all attacks.
- If at any time you receive healing from any source while Wounded, flip your board back over to the Normal side, carrying over your HP total (including those gained from the healing).
This would take the place of Healing Surges. Seems not to harsh to me, and still gives you options while being wounded.
Perhaps I might write up some Power cards that you could choose for your Hero that utilise the Wounded state, like the Fighter going into a rage, etc.
For me, Disadvantage on Attacks is still far too severe.
I would suggest an alternate method, it would save having to create a new Hero card with a "Wounded State".
Create a bunch of Wounded cards (they would effectively replace the Surge counters). When you would normally take a Surge, you randomly select one of the facedown Wounded cards.
The Wounded cards have something along these lines (each one is different, or the stack contains 2 or 3 copies of each card).
* -2 on attack rolls
* -2 on AC
* -1 on movement
* -1 on Attack rolls and -1 on AC
If you wanted to add more severe versions then you could add some such as...
* Disadvantage on your next 5 Attack rolls
* Next 5 Monster attacks against you have Advantage on the Attack rolls.
* Any damage you receive is now increased by 1 HP
There should be ways you could recover from your Wounded status. Maybe, when healed, instead of recovering HP, you are allowed to discard your Wounded status.
The main problem is that all these ideas are making more difficult (or just more frustrating to play). You need to have something that offsets the disability, such as your Power card suggestion. Mind, you can't make them too strong otherwise it might be seen as an advantage to being wounded.
The nice thing about the Wounded cards is that they are different and you won't know which one you'll get, makes it a bit more thematic in my view.
EDIT: I would also add that Wounded cards function as Healing Surges (i.e. you recover the HP as shown on your Hero card) and that you can obtain multiple Wounded cards during the course of a game (just as you can obtain multiple Healing Surges). The game can also ends when a certain number of Wounds have been taken.