Stacking those would be sweet, but not holding my breath

I think they won't do this because they want to incentivize eso+ for people, which is still capped at 1k transmutes but that gives you more room to work with than the non eso+ cap. Also if you could have unlimited transmutes (which is basically what you are asking for, except you're willing to give up a few inventory slots permanently for it) then people would stock up on them and never have a reason to do dungeons again to balance their transmutes unless a new dlc comes out, then they just get whatever peices they can and use the 50 transmute trait option instead of the 25 reconstruct if you have the full set. It's very much a game design issue and I think they would need to pass on this, as some of the players who do dungeons every day for transmutes during their active period may stop doing them, and people may do trials less, etc.

also obligatory "back in my day we didn't have transmutes, we kept farming the same crappy drops until we got what we wanted"

honestly eso+ should just remove that stupid transmute cap lol, its just a currency