What?
Allow to assign custom registrations to aircraft (adhering to the general formatting rules of the respective registry). While at it, it might be fun to see whether more realistic rules for registration formats could be implemented. For example, the first letter of the German registration marking actually depends on the MTOW of the respective aircraft (even though the vast majority of aircraft in AS would always get an A 
).
Why?
Often-requested cosmetic feature.
When?
No timeline, low priority.