phantoms under levitation drop elytra

when a phantom under levitation dies, it drops an elytra.

That makes elytra renewable, but still locked behind finding the first end cities, since levitation can be given only by shulkers, and they can be found only in end cities.

It involves bringing a shulker into the overworld, so still makes exploring the end worth while for a long time.

If considered too broken of a feature, you have a few balancing options:

 - make it so only *one* elytra can drop every night

 - make it so elytra are a *player kill* only drop

 - make it so elytra only has *a 5% chance of dropping*

 - any combination of the above

Why make elytra renewable? 

Elytra today can only be found through exploring the end, which on older servers already explored becomes harder and more time consuming, that's a problem for long time players / long lived servers specifically. So the solution should only be worth while for those cases.

 Why phantoms?

The phantom membrane is already used to repair elytra, so it's logical to assume they could drop elytra in some cases.

It gives phantoms another use, which is always nice.

Why does the phantom need to be under levitation?

It keeps elytra locked behind finding a shulker and bringing it in the overworld, so your first elytra will still most likely come from exploring.

For the same reason, slowing down the process by making it a player kill only, a 5% drop chance, or a one per night drop only balances it in favor of exploring the end for longer, before having to resort to this.