Thanks for getting back to these GPD!
I'd say "to what end" being realism as the gold standard. For instance, lets use a 5 round fight currently in the game as the "max stamina" reduction you can see.
If the fight
NEVER goes to the ground you will normally see fighters struggling to throw 2 or 3 punch combos by the end of the 5th because the stamina is reduced enough to make it difficult(maybe 25 to 30% stamina bar remaining). I feel this looks correct and realistic for stamina in the stand up game. Its still slightly on the "to many punches" side of things to achieve the desired results, but it is far more realistic than the ground game currently.
If you did the opposite of the above and kept the fight on the ground the entire 5 rounds, your stamina would be very high, far higher than it would be if kept standing(maybe 50 to 60% higher than the standup example).
It should be completely reversed, the stand up game slightly more forgiving while rolling around on the ground the entire fight should be sheer exhausting for both fighters.
I think it could be done in a couple ways if the ability is within the code to do it.
1) Is ground strikes and stand up strikes setup to take different stamina penalties or is all strikes whether ground or standing the same type of penalty(obviously type of strike I know is a factor)?? If so you can slightly raise the stamina cost to throw strikes on the ground. Which then leads into #2
2) Increase stamina cost for modified transitions.
3) Increase stamina cost for denials like you mentioned
4) Lastly, apply a stamina penalty if getting reversed.
I believe if you make slight changes to multiple areas so no one thing is being heavily changed you could achieve a more taxing ground game. I do believe from a gameplay perspective denials should be the most influential area to dictate who is taking the heaviest stamina toll.
Now, I also feel everything ground related needs to cost more on the ground. I think if anything this is my biggest point-of-view of the current ground game.
A stand up fighter, say someone like Cro Cop should not have a stamina advantage just because of his specialty. I say that with the mind set of this example. Say I have a ground guy who sucks badly on the feet. My strength would be to take Cro Cop down and avoid his deadly kicks. Now from a transition/denial area I might have an advantage of holding him down, BUT if he can work back to the feet, the stamina penalty I just applied to him by holding him down for say 2 rounds will
not be noticeable. Our stamina will be nearly equal. He should be slower from the taxing ground work of me making him carry my weight. Thus when/if it gets back to the feet, the stand up is little more in my favor. The current system, we will get back to our feet and both of our stamina will be nearly the same, resulting in him still having an advantage.
I think a good 50% harsher would be a very rough estimate for me.