Does Unearthed Arcana count as Core? It's in the SRD, so my group calls it "Core." If so, I'd skip specializing (and Red Wizard) entirely and go for a Domain Wizard. Many of the benefits of a specialist, none of the lost schools. If you want a prestige class, I guess you would be limited to Loremaster and Archmage, but honestly you might be better with Wizard 20 or Wizard 15/Archmage 5.
Just my two cents, of course. The Circle Magic ability of the Red Wizard is quite nice and versatile, so it's really up to you what seems more fun/versatile.