Sure a starting one would be to write a storyline for lineage and quotes / dungeons associated with that. Final fantasy, wow and so many other mmos show that this can be a really important part of the game, we all know lineage has some amazing lores and for those deep delvers we remember the pagan temple storyline which is a legit really interesting.
More importantly gameplay wise you can create amazing story driven dungeons and break some of the grind and allow you to play the game actively, plus they can be excellent story content, solo or group experience.
With unreal 4 for example you can make some amazing mechanics happen and games line dungeons and dragons online, have some of the best solo/group dungeon content there and ncsoft can make lots of money monetizing it while keeping player engagement, expanding the game and the lore.
Obviously this is a small fraction of some of the suggestions like a re-incarnation system, where instead of being stuck at level 50-60 etc. You could level up at a more enjoyable pace that maybe allows you to play more active when you have time instead of being grind stuck, and reincarnate. Lineage 2 already has so many passives a few others I imagine won't be an issue or even actives where at max level you reincarnate and get small boosts.
This way you diversify player content, allow them to be more pro-active when they want to, it makes lower level areas booming with people and does not make the game look empty, a mentor system would be excellent with it you can help clan members level up etc. You diversify people's gameplay and keep old gear relevant.
These are a few small ideas you can easily implement at very little effort in lineage 2. Diversify people's gameplay and make them feel less stuck in some aspects while keeping the power progression we enjoy about mmos in there.