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Crossed my mind, a system which could be implemented on an excel file...
Doesn't have to be Excel. You could simplify that by saying that penalties occur when a character drops 1/4, then 1/2, then 3/4 of their Health Points. The penalties could be a proportion of Attributes (as you suggest), or a penalty to dice rolls (either in Combat or checking a skill).
While it's realistic, I'm not sure I'm so keen on the idea. I find that one of the strengths of DW is its simplicity. You can pick up the book learn the basic rules in about 10 minutes and then you're playing. (It's simplicity means you can even run and play games when suitably...
lubricated. Not that I've every tried this, of course. Ahem.) I'm not too keen on adding every more rules, so that DW becomes one of those games where every combat results in players flicking through rulebooks (or consulting rules online) to see how a particular situation is resolved...
It may not be particularly realistic, but combat is already pretty dangerous in DW. No character ever gets many Health Points and it doesn't take many blows from an ogre's maul to see even the stoutest of knights fall over... Adding penalties makes the combat even more dangerous.
You may also want to consider this: every rule you make that increases danger is invariably a disadvantage to the characters. The monsters might be affected too,* but they only fight once: your characters will have to fight repeatedly to complete an adventure - so penalties accrued in one combat will affect them over several combats. Making wounds worse (and similar "realistic" rules) makes those adventures harder to complete... It depends what you
and your players want out of the game.
* Except the undead, presumably?