Backpacks: As more items get added to the server, which I'm sure will happen overtime, players inventories will begin becoming cluttered with items. In order to avoid any issues with this players will need new methods of storage. Personally I think the best way to implement this system would be to give all players a single backpack with 9 slots for free and allow them to be upgraded with coins to add more slots and possibly even extra pages. (Similar to how PC storage works, but with items instead of Pokemon). Enderchests could also be used in the same way, giving all players access to an enderchest that can be upgraded for increased storage. If you want to add more incentive's for players to buy ranks, increased storage size could be added to rank perks, however I think upgrading with coins would be much better for the server.
Seconded. I like the rank option and giving a purpose to the coins is a good idea as well. Each rank gets coins though..so the upgrades COULD be bought with said coins..if the player decided to use them for such.