As many know, there is only the one Dragon egg available in the game & you can only get it by killing the very first dragon in the End.
For those of us that play single player - this isn't a problem, but on Multi-player servers it causes a small problem.
How can players complete all the Advancement & get 100% if there's only the one egg.. The only way is to find WHO killed the first Dragon & ask them (very nicely) if you can hold the egg. But if this player has left the game/server, then the Egg is effectively 'Lost'
I'd like to offer a suggestion - A new Gamerule '/gamerule spawnegg xx'
What this does is causes the Spawn egg to be respawned when the 'xx' dragon is killed.
What do I mean by 'xx' dragon killed ?? - Depending on the value you enter means that many Dragons need to be killed before a New Egg is spawned.
(Setting to '0' mean there'll be only the one egg) so if you enter '/gamerule Spawnegg 20' - this means that after the First Dragon is killed (& the original Egg is spawned) then the next 20 Dragons that are respawned using End Crystals the Egg WILL NOT SPAWN but on the 21st Dragon Respawn (& killed) then a new egg will appear.
This way it'll give other players on the SMP a chance of getting a Dragon Egg & managing to 100% the Advancements
As a 'Bonus' maybe 'respawned eggs' can have a slightly different Egg graphic - maybe same egg pattern but with a different coloured speckled effect - so that players will know whether it's the 'Original' or a 'Respawn'