Here's a sweet idea:
Have Soul Gauge as a bar (that damn orb does not give enough info anyway). When you block it goes down but leaves a red trail similar to MvC damage. You can choose to initiate a Soul Charge by sacrificing the red, the more sacrificed the longer it lasts (very similar to TvC). Now the red trail will naturally heal away similar to SC4's current gauge, but sacrificing it reduces the max cap to where the red trail ends, making it harder to keep your SG gauge out of CF range.
So this is a combination of systems we've seen work. You can obtain an offensive buff (soul charge) by blocking, but must use it carefully. If you simply execute it when you have advantage, you risk sacrificing too much so it may be wise to let it heal some before using. If you use it after a small amount it will not last long to justify. Further more, trying to abuse it for offensive gain puts you at risk of CF.
This ability would not recover each round, making CFs very likely g3-5 and creating drastic strategy shifts over rounds.
I think this idea would appeal to a LOT of complaints and wants. You get the return of soul charging, you get rewarded for good defense, the comeback aspect is very balanced and is a double edged sword. This idea would be amazing in terms of strategic depth. Seriously, if Namco posts a suggestion survey like they did last time, please enter this idea.