I don't know if this has ever been suggested or discussed, but here's an idea I've just had regarding charged items.
An Invested Item (i.e. an item that has been prepared for enchantment using a season as well as Vim vis) can be instilled with a charged effect. Basically, the charged effect is designed just like a charged item, except that the number of charges is 1 for every 4 points (or fraction thereof) above the effect level. The effect does not require raw vis to instill, but takes up space based on its effect level.
This kind of item is of interest for vis-conscious covenant and magi, and is slightly more efficient (time invested vs number of charges) than the basic charged item. On the other hand you want to be more careful about the object itself, since it represents an initial investment in both time and vis, and it can be reused for other effects.
You can mix permanent and charged effects into the same item, which allow you to have a flexible invested item that changes in time based on the magus' needs.