the mirror modifier is one of the most useful modifiers in blender, but some time I miss some features that would make it more powerful,
this is my proposal:
Filter vertex groups (or exclude vertex groups )
add the ability to filter which group of vertices will be mirrored and which ones do not. eg:
I want to model a desktop, start modeling the top half of the table, two legs, but the drawers I would put in a vertex groups that they are only drawn on one side.
that could be useful to model characters who have diferntens thing in oposite sides, like, guns, wristwatch, earrings and so on. (it can be useful to make asimetric characters filtering the some parts and modiling it manually )
this is possible today using the option "mirror vertex groups" and adding a mask modifier over groups of vertices, but it would be great to integrate it into the mirror modifier since the mask modifier reduces the performance
add an option that allows after finishing the modeling, create off set in the vertices,
eg: we model the left side of a character, the modifier generate a kind of cache, and after the binding we could edit and generate asymmetries only in the Mirrored mesh.
(sorry about my bad english)