To be honest, the server can't go -that- much further when it comes to official content. Not sure if you guys have full 13.2 yet, but starting from Scaraba Hole, it will get tricky to update. Gravity fully switched to Renewal on all their servers, so anything after 13.3 will have Renewal stats and Renewal drops. Sure, you can add them with custom stats, but the question is if you want that.
eAthena released the official Nydhog Instance a couple of days ago, though. So adding that should keep people busy for a while.
Custom content is IMO the way to go. People can find official features on any private server. It's new stuff they want to see. Minigames, unique events, just... stuff to do apart from the endless grinding. Also stuff that will keep old players busy. Some epic challenges that will require them to use the fullest of their gears and skillz.