I would look at the races of stone substitution levels for a gnome illusionist. They are, in my opinion, the best racial sub. levels out there. The 1st level option effectively lets you cast all the major illusion spells as a whole level lower - ex. major image becomes 2nd level and silent image becomes a 0 lvl spell. The drop in spell level did not effect my dc's too often since the illusions would, more often than not, not need to interact with the targets.
The 5th level sub level was good too. Free double duration on all illusions you cast helped in several situations.