It would be useful to add custom icons for investments when there is no official icon yet available. Also some “official” icons are very blurry as the resolution is bad. (see the DEKA icon for example)
This could be implemented with a limit (e.g. 20 custom icons total per user) for avoiding abuse.
Also in addition to that it could be useful that users could “submit” an icon and thus help building the icon database quicker than it is today.