Well, I was thinking that you could have the DC start out at 20, raise by 2 for every character level you have, and raise by 1 per circle of the spell you're casting. There could be other factors too. So a level 20 sorceror casting a 9th circle spell would need to beat a DC of 49. In other words he would probably need to invest in Influence, buff his charisma, use glibness, and wear some influence gear if he really needs to cast in public. Then if you want it even more difficult, that's where the other factors come in. Like every spell you cast in a row (1 minute cooldown?) adding +3 to the DC as you battle to tame that "maelstrom of power."
I feel like this sort of freedom, allowing players to spend points in an otherwise not very useful stat, but gain some utility for it in the rare case he needs it, can only help with character individuality.
As sorcerors must remain mindful about casting but now have a mechanic to avoid being outed immediately, they might mention their differences to traditional, inoffensive wizards more. Since it's a rare class as is, we can't expect them to have good examples of PHB-abiding players out of the gates, because there is no mention of the PHB only lore unless you go digging, and not everyone bothers doing so. But if confronted with this mechanic, and the sheer difficulty of it, they may inquire with others and read up as to why it exists, rather than just surrendering to its uninteractive nature as it is now, mentioning it once as an off-note, "I'm a sorceror I can't do magic in Port" but consciously investing skills & money into gear so that they can do that -- whenever people model their play around something they usually take it quite seriously and mindfully. Wishful thinking?
edit: fixed an obvious error