While something like this may be kinda cool for less-skilled players, it's completely unnecessary. When a beatmap is designed, it takes into account how fast your health drops based on the amount of combos (or so I've heard), so if there were a special "power-up" or something, it would make the map much less fair to those skilled enough to handle the map. Not saying you aren't skilled, just saying that as you get better, something like a "health boost" wouldn't be needed.
Some of the "challenge" maps (I.E.: Marisa, Tsurupettan, etc.) have very low health drain. This is because of the sheer difficulty of the songs. Why make the drain low? So more players can pass it. If a map's drain is high, it just simply means that you need to have accuracy. If you don't play accurately, it's best to keep away from the song for a while.
So yeah, that's my take on health-boosting circles.
As a side note, a health boost in this game would most likely not even be a hit-circle. If something of the sort were to be implemented, it would most likely appear as the well-known plus sign, and probably be 2 circle sizes smaller and only be on the map a few seconds. It wouldn't give a score boost-- in fact, it might actually lower score. This game isn't really meant for power-ups and health boosts... or at least not traditional gameplay. Maybe you should propose this idea for a different mode of play? *shrugs*
