How about stealing from
4e?
All characters have a base F/R/W save equal to half their level, rounded down.
Classes give a class bonus of +2 to one or more saves. E.g. a Fighter gets +2 to Fort, a Rogue gets +2 to Ref, so a character with levels in both Fighter and Rogue gets +2 to Fort and +2 to Ref; a character with levels in Wizard and Knight would only get +2 to Will, because like bonuses don't stack.
Humans get +1 to all saves.
Note that this gives the single-classed character the same good save values at Level 1 (+2) and Level 20 (+12), and neatly segues into Epic.
Edit: BTW Fighters who multi-class into non-full BAB classes really get borked at Epic levels unless you allow Epic Prowess to count towards EAB.