Since completing a collection of 15 different eggs requires either a lot of AD or cooperation between players ....
Why not have the 'reward' be a Voucher that will automatically grant the user the ability to upgrade their Stronghold Guild Hall to level 20. The 'Guild Hall Rank 20 Upgrade Voucher' (tm)
This would have guilds and alliance members working together to gather/donate their eggs to benefit a group of players and not just one in the true spirit of an MMO.