I don't know much of what is happening with Sigma but thought I would suggest changing the entire concept of charms to be more in line with how you described wanting to have SU be build enabling and SSU be build enhancing. Maybe have UBERS and the like each tierd (group of easy ubers, each having a chance to drop a charm with random Oskill, then group of harder ubers dropping charms that enhance certain skills in a minor way, and then the hardest ubers charm enhancing skills more dramatically).
Each tier of Ubers/challenges could have a single boss per class that drops their charm so a player is not required to kill them all but still has the option to kill them all for the fun of it. Alternatively if the goal is to make all of them mandatory it could be done via a cube reagent each one drops that when cubed unlock the next tier. This could potentially create more build variety and make advancing threw the game smoother by having fights tailored to specific classes/builds (and not single fights mandatory for everyone. Would also eliminate the generalized power boots that the current charm system offers.
Just my thoughts after reading a little bit of whats to be expected in Sigma.
PS. Love the way median has progressed over the years! Thanks for all the awesome work!