So for reference:

• www.dandwiki.com/wiki/SRD:Spell_Focus
• www.dandwiki.com/wiki/SRD:Illusion_School

Spell focus gives +1 to save DC for the chosen school, in this case illusion. Looking at the spells listed under illusion school and filtering on those useful in combat we have:

• lv 1: color spray: unconscious, blinded and stunned, will save negates (similarly hypnotic pattern and their higher level variants are all great crowd control spells)
• lv 4: phantasmal killer: will disbelief, then fortitude or die (3d6 on successfull save)
• the series of www.dandwiki.com/wiki/SRD:Shadow_Evocation and shadow conjuration and all similar spells are pretty good too, they allow you to cast any other evocation/conjuration spell of lower level, at the cost of an extra potential will save to disbelieve (but also at the DC of the shadow spell, which is higher level) (insert “mom says we have heighten spell at home” meme)

And that’s just at first sight a couple good ones.

Sure, you won’t be throwing around +1 save dc fireballs with this (except shadow evocation gives you heightened fireballs, at the cost of an extra will save), but CC is often more useful than direct damage. Especially out of combat (depends on what you mean with “roleplaying aside”), there’s lots of good options: hiding behind a silent image is a cheap mass invisibility spell, or distracting guards with ghost sound for sneaky infiltration.

Good luck!