like I've mentioned in a different thread, i think the answer is creating more content. doesn't have to be anything large scale like the new expansion packs, and it doesn't have to be epic. just keep creating small adventure packs for all levels, and people will buy them.
why would they? because if you're a new player and your party is now about to run tangleroot and you don't have that pack, at first you'll simply drop and find another party. after 3-4 times that this happens, you'll really want to buy that pack...
and if you're a veteran you'll really want to buy that pack just to have some variety when leveling.... TRing can get tedious.
the problem with the expansion packs is that they push the cap further away. essentially making them completely irrelevant to f2p players and to veterans who are working towards a heroic completionist.
so it's a massive developmental effort and investment, going mostly to waste.
making TRing harder isn't the right way to go about encouraging people to play those epics. IMO.