I don't think anything should be remixed except for AJFA because it has a glaring problem accepted by the majority of Metallica fans and the band.
For better or worse, the rest of them are simply products of their time, and their sound is part of the character of those albums. Personally I think KEA sounds great for that era of the genre, and MoP sounds good too. I'm not a fan of RTL's dull sound, but there's not really any objective problem that needs to be corrected. It is what it is, so I'd want that left alone too.
I definitely don't want them to go as far as making artistic changes to the sound of the album. They just need to boost the bass a little on AJFA, and perhaps EQ the guitars accordingly for the mix to work. They shouldn't touch the reverb, they shouldn't mess with modern production techniques and ideas, they should just finally do justice to the album and make it sound in line with their other classic albums. Change it as little as possible to correct that one problem.
Chances are they're just going to remaster them and be done with it, and we'll be let down. The main motivation for re-releasing them is because they now own the recordings and want to make money off them.