Yesterday I talked about ways to limit the enchantments for armor, basically by some trait of the material.*
Today, let's talk weapons. What if you want to limit the maximum enchantment by underlying weapon?
First, there are a lot of weapon enchantments in GURPS Magic. Flaming Weapon, Icy Weapon, Lightning Weapon, Penetrating Blade, Puissance, Accuracy, Loyal Weapon - just to name some off-hand.
Second, some of them already come with restrictions - specific materials needed, or specific construction (no Flaming wooden weapons, and all icy weapons must mount a specific gemstone.) The below are in addition to that.
One Level Per Level of Quality - this draw a line in the sand at good quality weapons. A baseline, off-the-tables good quality weapon can one one level of an enchantment. Puissance +1 at most. Fine allows a second level, or Puissance +2. Very Fine is the only level that allows Puissance +3. Cheap weapons simply cannot hold an enchantment. Accuracy might work just fine for one level on good quality weapons, but need Balanced before it can be +2 or +3.
One Level Per Modifier - For every weapon modifier, it can hold one level of enchantment per enchantment. You can push this further by saying one level of enchanment, rather than one level per enchantment. Want a Puissance +2 Accuracy +2 Flaming Sword with Penetrating Weapon (5)? It needs eight weapon modifiers or levels! Bring on Orate +3 and Very Fine, you're going to need them.
Limited By Material - Unlike armor, weapons can come in a variety of materials without changing the base type. So limit enchantments by the underlying materials. Only metal can be enchanted past +1. Only silver can hold Ghost Weapon. Only steel weapons can be Penetrating Blades. It can be even more exotic - the Malazan books limit the best swords to those made of stone. How about that for flavor? Excalibur sure is nice, but it's not made from rock, so, well . . .
This can also serve to limit especially cost-effective spells, like Penetrating Blade. If it can only be used on weapons of a specific material that is otherwise limited in enchanments, expensive (and/or rare), has odd drawbacks (can't hold an edge, only a point), it suddenly becomes a niche enchantment instead of a baseline need for all fighters.
Limited By Specific Modifier - pick one modifier (Fine works) and limit each (or all) enchantment by that modifier. If you need Balanced for Accuracy, Fine for Puissance, Elven for Quick-Aim . . . you end up with weapons that have a design bias (or cultural bias) towards certain enchantments.
Cost Based - like I mentioned with armor yesterday, just set a ratio. Swords suddenly become much more likely to be enchanted because of their cost. A Dwarven axe is nice, sure, but it's low base cost suddenly holds it back from being the most powerful of weapons.
For any of these, you can make exceptions based on things like the Named Possession rules or for artifacts or other special cases such as holy weapons. But there is a real charm to needing specific ingredients (or specific construction) that drives what magic a weapon can hold.
* One poster on G+ called that a solution in search of a problem. To be clear, this isn't something I think is a problem, game mechanically or otherwise. It's a setting decision. Sometimes you want a setting where people to turn everyday trash swords into Excalibur with magic, sometimes you want to limit it to the best materials. Or tie specific enchantments to specific materials. This post and the one yesterday are for that; they're flavor changes, not game balance "fixes." It can really add some flavor to the game when people have to hunt down rare materials in order to allow a really powerful enchantment.