Something offered by the setting of Rohan is its expansive lands. Certainly LotRO has some massive zones but within a country dedicated to the art of horse riding, it seems only reasonable that it should have some of the widest, longest zones in the game. Obviously they can be too large, for fear of travel time that takes well.. ages. However, with improvements to a player's riding speed -- or just the fact that they have a horse -- it shouldn't be much of an issue.
What's important about the addition of a wide, expansive land in LotRO is that it appeals to a wholly under served type of MMO player: the explorer. An open and vast country like Rohan offers an almost wild west kind of appeal. It's an untamed land that begs for exploration or at the very least travel.
I've mentioned PvP and the open lands of Rohan, which brings us to the inevitable discussion of open PvP. To do this, however, Turbine would need to readjust its method of creating a PvP character -- potentially making it more robust. While I don't claim that such a change could be preferable, I do know plenty of people who would become considerably more interested in LotRO if it were to expand the "monsters" side of play into something, well, more.
In Rohan, evil forces are beginning to roam the lands. Now, these could be AI controlled creatures, but if they were players it would be so much more exciting. Not all of the roving war bands need to be players, but why not allow it? PvMP would get a nice shot in the arm while keeping the game well within the realm of acceptable Tolkien lore. Now, as for the actual implementation, I can't really offer any suggestions. I'm no developer and thus I do not make magic happen; especially when it comes to the kind of programming and number crunching something like this likely involves.
Still, I think it's a wonderful idea and if made into reality would easily attract several new players into LotRO. I'd certainly play a whole lot more of the game, which is saying something for a sci-fi fan like myself.
There's really not much more to say, because the fact of the matter is that LotRO is already a pretty great game. Sure, these things I've mentioned would be great improvements -- at least in our eyes -- but ultimately the additions and changes that Turbine makes are up to them. Things like legendary weapons and player housing were very cool add-ons, so whatever the developer does, I'm certain it'll be something worthwhile. Until the announcements are made, however, all we can do is ponder the possibilities.