These are all great ideas, but there's a key data management hurdle here. As I understand it, Z has each player's device locally storing each kind of plane's details within the app software itself. The zebras were actual plane type additions, added when he sent out a new release.
To keep the multiplayer game playing fast (or at least, playing not any slower than it is today), there is a design rule that minimizes the amount of data that needs to be transmitted to each player's device by a server, and to the server from the player's device. Up to this point, this data is about flight position (and yes, plane type, but with so few plane types, that data can be fit in very small number of bytes, sent when the player entered). That's how we see each other in the right positions (and the plane type databyte tells each player's device's app which plane to represent for whom in that game). Computational declarations of kills or bomb deliveries are all done locally on each device, then sent to the server, and distributed to the other players' devices. At least that's what I assume, based on a little bit of what Zup's said, and a lot of assumptions on my part.
If we were to add "custom" planes, meaning each of us could come up with a custom design, even if it's just a logo under the cockpit, this data would have to be sent to the server every time we entered a game. Then the data would have to be sent to each of the other players as they enter a game with you. That's an enormous amount of data. Data transmission about plane positions would slow to a crawl. We would all be ~~~oo~~~!
Unless of course you don't care for anyone to see your customization except yourself. This would require no data to be sent, if everyone would see you simply as one of the "standard" dogfight plane types with none of your customization showing. We could show off our customizations on the forum. But would this be fun enough to warrant Zup having to add Painting and Pasting capability to the DF app?
Manfred