Meh. You had me there. Actually i wasn't thinking about directing water inside ogranisms on order to forcefully move them (like a hydraulic tube) - which would make fleshbending a special case of waterbending (cue full moon). My bending would directly influence cells to perform stuff like mutation, division, secretion etc, with a visible and orderly macroscopic effect.
Yeah i know that's waaay out of line compared to other benders
It would also kinda make me a mary sue (or rather.. gary stu). Since there's no indication of this particular sphere of bending existing in canon. So wery well. I will step down and abandon this altogether.
Actually, i have a hunch that we're already stretching it pretty far by allowing steam, smoke & sand bending. It's not fun if everyone is speshul and unique and a one-in-a-million freak occurence. I personally think we should limit your ELEMENTAL AFFINITY to exclusively ONE sphere. That's it. One. Uno. You can either be a fire, air, water or firebender. So no weird 50-50s or 15-25-60s. However..
What's stopping you from pursuing a very narrow specialization in your 'sphere'. Or several. And what's there to say you won't eventually, through character progression and development, find your very own unique style within it. Much like fire has lightning, earth could have, say.. sand. That doesn't contradict canon. It complements it.
About the multiple elements.. only the avatar can do it. Your affinity is bound to you. You in essence (your spirit) is what presumably allows yourself to attune to a certain element and affect it. No science involved (remember the MST 3K mantra. It's just a show. Relax. If something unexplained happens well.. there's probably umm.. a wizard. Somewhere. That's responsible for it). You could, i guess, forcibly train yourself in a secondary element with vastly diminishing effects. Think that's lame? Well broseph..
You don't need to master all the elements. That's what TEAMWORK is for. Hint, hint.
I was also thinking of implementing a rolling system, if you guys agree. Will detail it if there's sufficient interest. Reworking my character..