Hardly balanced when comparing to existing effects: A weapon property giving +1d6 energy damage plus additional 1d10 on critical hit is +2. 2d10 on every hit would make existing elemental weapon properties obsolete. It would also make them too powerful when comparing to other existing weapon properties - it could be house ruled but I'd advise redesigning weapon properties to match this change.
Also small terminology nitpick: in D&D 3rd edition and Pathfinder enchantment refers specifically to school of magic dedicated to mind-affecting effects, not the process of creating magic items. What you are speaking of is weapon enhancement.
If you are the GM, Lou, you're free to do as you see fit. If you think it's necessary for what's being run at your table, do it. Sometimes its fun to do what isn't necessarily "by the book."
For instance, one PC at my table (who isn't aware of it yet) will eventually come across a modified Sun Blade (specific weapon) to help him fight off some particularly rough monsters.
Imagination comes first for this kind of game.
Enhancement. A weapon enchantment would serve to confuse, bewitch or befuddle your foes and not do any elemental damage.
So, let's talk balance. For kicks, I'm going to call the OP's idea "super flaming" and assume it's only flaming damage for now.
I could get a +1 flaming sword for a +2 effective enhancement cost. That will do +1d6 damage on every hit. I could get +1 flaming shock which would do +2d6 on every hit. I could get +1 flaming shock frost which would do +3d6 on every hit, as a total of +4 enhancement.
That's fairly close. Let's compare:
+1 Super Flaming sword does +2d10 on each hit, averaging 11 extra fire damage. My +1 flaming shock frost sword does +3d6 on each hit, averaging 10.5 damage, almost the same. As a side benefit, when fighting a red dragon (or any creature immune to fire), my sword still does the shock damage (and in many cases 50% extra frost damage), but the +1 Super Flaming weapon does no bonus damage because it only has one elemental type and the enemy is immune to it. But, on the other hand, many more enemies will be immune to at least one of my three elemental types, so I frequently will be unable to do all my 3d6 damage while the guy with the Super Flaming sword will only occasionally be unable to use his extra damage.
All in all, that sounds like it could be a fair trade off. I would use a +1 Super Flaming sword against white and blue dragons but I would use the +1 flaming shock frost sword against red dragons, and I would use either one against green and black dragons.
Which says to me, +3 is a reasonable adjustment for this ability.
(And that's true of any elemental type, not just Super Flaming)