I think you may be overstating how powerful irrestistable damage is. For instance, this spell would be weaker than Fireball, because there are more creatures who could fail the Reflex save than there are creatures with Evasion.
Undodgeable Flames
Level: Sor/Wiz 3
Save: None
As fireball, except as above, and Undodgeable Flames does 1d6/2 levels damage (max 5d6).
If you reduce it to a single creature and make the damage untyped, it will again be weaker or equal, as there are at least as many multi-foe encounters as fire-resistant encounters.
So already, we're well on the way to an unresistable spell, and if anything getting weaker rather than more powerful. It's not an impossible concept.