Actually, there is a ton of spells that simply can't be improved with more caster levels because they have a hard cap. The earliest example you can find out is Protection from Element, that caps at caster level 12, for protection against 120 elemental damage.
Since now we are reaching epic levels, and we have at least one precedent of hard cap removing (Stoneskin used to cap at 15), I think we can add more layers to some spells.
- Protection From Element and variants lose any kind of cap. With the sheer potency of some EE spells, having a 350 points shield usually blocks 1-3 spells before disapearing.
- Resist Energy should count for epic levels. Make it raise up the resist to 40 with a caster level of 21 (basically any pure character in their home sphere), and 50 with a caster level of 31 (requires a ton of effort, or savant in arcane sphere).
- Mage Armor originally provided protection simmilar to a chainmail. It should provide protection on par with equivalent armor. So, +5 at level 4, +7 at level 10, +9 at level 16 and +12 at level 22. Unlike armor, however, it still don't give you any PRR.
- Shield should provide a similar upgrade, since it provides the same bonus as a Tower Shield. So, +6 at level 4, +8 at level 10, +10 at level 16 and +12 at level 22.
- Flame Arrow should work more like Conjure Bolts. Make it invoke 1000 ammunition/throwers at once, and make their enhancement bonus raise up with caster levels in the same way.
- While I can understand the hard cap on Heal and Reconstruct (specially after the changes to Spell Power mechanic), I think Harm and Deconstruct can be seen being cast a lot more if their hard cap is removed.