Copy leveling experience of gw2, majority of mmorpg players thinks this is the best and most fun leveling experience atm.
For example - in starting human zone, every 15min there is an event you have to kill 50x fungus, 50x wolfs to spawn main boss. After an event you get exp to level up from 1-5lvl and so on. With this system you also show new players how the endgame will look. Also give them some special currency from this events to buy some item shop stuff like madox mentioned.
A must thing that is currently missing is also WORKING GROUP FINDER. spending 100x megaphones to form a raid is outrageous.
This would also help new players to form parties for zone events.
Example - look at ffxiv system and copy it
Make it so pvp arenas give exp orbs to level up just like in pve.
Make some kind of catch up system. Especially for pets/cenedrils. You cant expect ppl to play 1 year just to catch up to old players, they lose interest in the process and leave the game.
To be honest, its going to be hard to get new players in such an old and outdated game.
If you have the legal rights for this game, just rerelease the game at some point, this would bring massive amount of players into the game.