With the advent of geomancy for aspecting the local background count, would it be possible for an advanced form to increase or decrease the local mana level? Granted, I'm not really sure what you could do with it, but inquiring minds want to know.

Hmm, well, historicaly it has taken ridiculously high-powered shit to raise the magic level, like the Great Ghost Dance of Aztlan blood magic tomfoolery. I'm more comfortable with aspecting the background magic, as is right now, than being able to outright raise it.

Actually, Toxic mages can already increase the background count. The taint metamagic allows them to increase the background count in a domain that is already aspected toward toxic magic. Of course, if they also have geomancy, then they can take any area with a background count of 1 or more, aspect it toward toxic magic, and then intensify it.

You'd be better off dicking around with some sort of manafont metamagic, internalising mana to a certain degree. Initiate grade nullifies a point of background count/mana ebb/whatever

Or you could play a taint aspected mage, and take the metagenetic negative quality that makes you a rating 4 tainted domain. (yes, that would be really cheesy, and your GM should slap you if you try it.)