My D&D DM says a Rapier that has your option of elements is broken.
Lemme see if I can find a middle ground on the Rapier. (I want it as my Legacy weapon, durnit!) More balanced now?
Incremental, Elemental Keen Rapier of Antipode II
Legacy Weapon, Cannot Purchase (No one short of a high-level street mage with a sense of humor would make this anyway.)
1d6+(Lv/5, round down) Damage
1+(Lv/5, round down) To-Hit Bonus
Threat Range 15-20
If Wielder is 5th Level or Higher, on command, Rapier is sheathed in Fire, Ice, or Lightning, (One at a time) and deals an additional 1d6 of appropriate elemental damage.
The first element used is its default element (For that wielder.). After that point, other two are only accessible 2x per Day (each) at a cost of 2 HP Damage per activation. (Stock up on Cure Lights!)
If Wielder is 15th Level or Higher, can use two elements 1x/Day (With no damage taken) for a total additional elemental damage of:
Fire & Ice: 2d6+2
Fire & Lightning: 2d6-1
Ice & Lightning: 2d6
If Wielder is 30th Level or Higher, can use all three elements for 3d6+1 add-on damage, and 1d6 backlash damage PER ATTEMPTED STRIKE.
Dispels as a level 25 spell. After a successful dispel, Rapier acts as a Keen Rapier. 3 days later, abilities regenerate at the rate of 1/day (DM discretion or Random Roll)
If dispel is successful by a margin of 10 levels or greater, Rapier deals 1d6 multi-element damage to wielder and 1d4 damage to all within 10 feet, and is destroyed.