I would like to suggest that spells like BES and CS become a 'gulping' spell instead of a held mana spell.
For example: We would prep the spell and cast it. Once in effect, lets say it only lasts about 30 seconds at no bonus or a minimal bonus to us until we harness mana. Lets say that we choose to harness 40 mana. The harnessing process, right now as the system stands, takes 3 seconds. Perhaps after about 7 seconds, the mana would be swallowed into the spell leaving you with ~0 held mana~ and the effects of the spell increased. We actually saw a portion of this in the old magic system when we ~added~ mana after our original harness, and I would like to see this ability to feed the active spell stay with us. Those are the trademarks of those spells in my opinion.
As for the negative effects of it: Perhaps if we harness too much(by feeding the active spell), we will start feeling queasy and begin to get a stomach ache giving us an internal abdomen bleeder.
Another idea, but perhaps more far fetched, is creating a verb called DIVERT which will allow us to divert mana into a particular spell giving us the choice of which spell to introduce our held mana into. As a side effect of DIVERT, I was thinking a small portion of fatigue should be used, but maybe a diversion pool could be made instead having only a certain amount of diversion in an alloted period of time.
Not only would this solve our problems with the held mana spells, but it would help keep the uniqueness of those spells.
Muely