1) give metamagics to sorcs with no increased casting time.
2) new skills as others have mentioned.
3) I leave HD alone, I think its fine.
4) One idea I've thought of using is similar to a MOTW idea. THey have a ritual where you spend 200 xp and upgrade your animal companion. WHy not have an xp cost per spell level for a sorc to learn a new spell?
Basically, there's a ritual that sorcs can undergo, to attune them with the magics of the world (which is in someways what they do now) and infuse themselves with new magical power. It should be expensive (money) and cost a lot of xp (Y amount per spell level maybe spell level squared).
This allows the sorc to get some more spells like wizards, but it should be much much harder for them to do so.