[SuedodeuS]

No deflector shields might actually be a good idea - a smaller fighter is harder to hit, whereas the larger ships that otherwise dominate can be liabilities. Alternatively, deflector shields that small, agile fighters can fly under could increase the effectiveness of fighters - or at least increase their necessity as interceptors for taking out the small bombers. Something that prevents massive numbers of mounted weapons on large ships can also be important, as they'll be less capable of defending themselves against fighters. Something allowing for aircraft-like maneuverability in space (as seen in most space opera) can also add to the "small, agile" appeal of the fighter. Dropping the effectiveness of missiles (like large amounts of PD that can easily take out missiles, but can't do squat against fighters) and beams could make the fighter more useful - if guns dominate, it will be difficult to defend against fighters (since lasers are ineffective against the fighters, and guns have to take into account their high velocity) without using one's own fighters (which can presumably drop in behind an enemy fighter and match velocity to get rid of the speed penalty).

These are just a few off the top of my head, and several (or all) of them could be completely wrong.