Kinda random idea. This'll be probably familiar to those who played Dark Cloud, as it is kinda like the Status Break/Synth Sphere system.
Anyway, on to the suggestion.
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The idea in a nutshell: turn equipment that are at maximum heat level into "modules" that can be attached to other equipment for bonuses. The exact bonus depends on the item in question (being a fraction of the item's stats/abilities/UV etc). The module is un-bound.
Limitations:
- The equipment has to be at heat level 10 to be turned into a module (alternatively, the % of stats transferred depends on the heat level).
- Turning into a module requires CR and energy, and maybe mats based on the * value .
- Modules can only be used by equipment of the same type they were made from.
- Some limitation on the number of modules a weapon can have (having to craft slots into the weapon for a price based on the * level of the weapon?).
- The Module replaces the UV (but if it had an UV it carries it over). (?)
- Once equipped, the module fuses to the weapon and can't be un-equipped, and is bound.
Why?
Having a max level weapon is a dead end, and in case you are tired of it it also can not get un-bound and sold. At the same time, there is a rather large amount of weapons that are dead ends as soon as 2* (cautery sword? Super slime slasher?). This gives these lame weapons a purpose (cheap way of boosting other weapons with abilities to boot). Finally, this enables further upgrading weapons without actually adding new paths, so it saves time on your side.
This would also be a great way to sink equipment without selling it.
Rather than requiring modules to be heat level 10, why not make them scale up in value by heat level? Level 1s get you a small boost, but if you're going for maximum value per slot you'll level them up to 10 first.