Let arcane casters cast spells like "Elemental Weapons" and "Keen Edge". ("Keen Edge" doesn't exist in DDO, I know, but I think it should be added.) This wouldn't be overpowered, and it would allow melees to drop Improved Critical in favor of another feat if they so wished.
(For those who don't know, "Keen Edge" is a spell that gives the same effect as the "keen" property, but doesn't stack with it, nor does it stack with improved critical.)
This wouldn't affect artificers too much. Let them keep sole posession of the spells that add material properties to a weapon. It's part of their niche, after all, along with their potentially infinite ranged dps.
Giving weapon buffs to wizards and sorcerers would also make a melee caster build more feasible.