Assuming we could get this work (and there's no guarantee there's a way to implement this in Bukkit), how would it work if the player is not gliding? Or if the player does not have elytra equipped? Which hand should the right click be with? Should you be able to specify the speed of the launch (i.e. more than 1.65)?

Should those questions throw errors if not successful, or should it return a boolean if the launch was successful? Maybe return a custom POJO that holds the return data?

There's several variables here that I'd like to be worked out before this is considered.

No, fireworks move the velocity closer to ~1.65 over their duratiom as well as creating a particle trail and finishing with an explosion. Regardless of whether you like the suggestion or not, sating that all it does is increase their velocity is not true.

All fireworks do is increase the velocity of the player while they are gliding. Not sure this method would be of great use, nor is it all that necessary in the current API as it merely wraps an existing API