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Randomspawns
Protected land (with the golden shovle)
IVS / EVS train / check
Auctions
4.2.7
vote shop | ability to vote
Random legendaries
Shops
All the shiny glimmering features like drop parties, tournaments, safari zones and all can wait - priority number one is to have a team that will help you along the way. Making a server is pretty darn hard.
If we are talking features though, the essentials ones are: an in-game shop, a webshop (donations to keep the server running), gyms (player gyms rather than npcs), balanced permissions, a good town system (to claim land with friends) and, as explained before, dedicated staff.
After that, you can look into all the other features you can add - but with that list, you got yourself a pretty solid Survival Pixelmon Server.