I like that idea... increase the activation TN based upon the number of abilities active (so the more you have active, the harder to activate another). The actual numbers will be more based upon how the abilities end up, but I am thinking +1 to the Will Save for each ability active (i.e. if you have none active, and activate 1, there is no minus, to activate a second, that is +1 to the TN, if you have 3 active, that is +3 to the TN, and so forth). This would allow for just requiring a simple Will Save (TN 15) as the basis for all psionic activations...
I was also thinking of putting in a negative modifier to physical actions while focused on concentrating... (say -2 if using half of your MC; -4 to all physical actions (including throwing that telekinetic bolt) if your using all your MC (perhaps I should simply call it "Psionic Focus" -- I do need a better name)
As for the Mental Capacity(/Psionic Focus), I was figuring the following
Phrenic (pure psionicist) = 10 + Will Stat
Adept (semi psionicist) = Will Stat
Wild Talent (non psionicist) = double Will Stat Bonus OR 5; whichever is higher
Psions would then have a BASE MC cost of 1-5 or 6(depending upon their basic strength). THe MC cost can go up based upon certain factors (i.e. number of targets, or increased range) These scaling options would not affect activation, but amount of MC required to activate/use them.
At least, that is my thinking at the moment... I am still going through my list of psions (I have over 150 possible psionic abilities at the moment).