I would be willing to bet that they have considered it, but also that they don't have the resources to really produce that content specifically for the weapon questlines. In a perfect world, I think everyone would love to have a questline full of nothing but new content through each expansion, including the developers themselves, but the necessities of making that happen are probably not within the realm of possibility for the team as it currently stands.
So since that's not something that's likely to happen, let's take your other thought: how do you make the relic questlines "more interesting" while keeping in mind certain fundamentals that the content should address: 1) it needs to focus on time investment as opposed to clearing difficult content; and 2) it should serve as a means by which older content can be made relevant again for the benefit of newer and returning players (or, if you would rather they not do this, you have to understand that development time would thereby need to be spent on finding a way to keep that content relevant so that they game doesn't stop attracting new players entirely).
I rather like the idea of NMs (I was always fond of them in FFXI), but the game already sort of has a model for this in Hunts, which would mean revisiting old content again, which you consider problematic. Whether they used NMs or Hunts (which are, essentially, NMs without unique loot drops and currency drops instead), over-reliance on them would also introduce elements of randomness and competition (hooray spawn-camping) that might not sit well with some players.
Doing some spawned monsters could be interesting (not unlike the NMs in FFXI's Abyssea), though. They did something like that to a certain degree with the Novus stage—some of those maps were actually a bit risky to complete solo. I wouldn't mind seeing a stage similar to that again (dress it up differently, though, in my opinion—don't just do maps again).