It might be interesting to do the 3e Fort/Ref/Will saves but let you apply the higher of two specific stat mods to the saves (Fort = Str or Con, Ref = Dex or Int, Will = Wis or Cha). It'd let you apply any attribute to a save as in 5e, and reduce the multiple-attribute-dependency of some classes by letting you get better saves from multiple builds, but you wouldn't have to worry about trying to work effects into the game that require six kinds of saves, and it'd be easier to balance the saves against each other.