Posted in t'other thread, but still equally relevant:
Yellow code letters and numbers for WWII Royal Navy Fleet Air Arm second line stuff. No one does any.
Post war Mosquitos and Sea Mosquitos. In addition, a set of markings in 72nd and 48th featuring the codes from "633 Sqn" could be rather fun.
Sheets of serial codes for cancelled aircraft - TSR2, Spiteful, Seafang, etc. Plus cancelled batches of Spitfires, Sea Furies, Tempests.
That's all off the top of me head for now.