What WitN needs is challenge maps that scale to your level (so the Lorien/Osgiliath challenge maps are still very challenging at level 40) and maybe some new challenge maps. If they also add loot to the challenge maps and maybe a very strong farmable boss at the end, it would give us a lot more replay value. Also, welcome to the forums!
As for WitN not welcoming new content, I think it would be easy to add new challenge maps to the map (I suggested Mines of Moria, including the bridge of course, and The Black Gate).
A level increase with new content would be a lot more difficult to make and it would also take time. Perhaps for now it's best if they stick to smaller DLC's, like challenge maps or cosmetical DLC's (item add-ons). A farmable boss, like a dragon, would also be a simple but nice addition that would give us more reason to keep playing.