Good, accurate subtitles. RoG's subs seem to be OK (and props to them for translating the Russian dialogue -- NOT the Japanese subtitles), but I think the remaining six films were either dubtitled or had new scripts which were primarily adapted from their respective dubs. These films totally deserve quality brand new translations.
But that's it. A prospective Criterion Heisei Godzilla box set would be an absolute pass for me because I already have all seven of those films via their Toho Video releases. I can't imagine that, besides subtitles and an Alex Cox interview, Criterion would include bonus features that aren't already on those discs.
OH! Criterion might also include the mono mix of RoG's dub; Kraken offered only a new 5.1 remix. (However, if the Showa collection is any indication, dub audio from Toho will be lossy, so this is probably a wash lol.)
I also wouldn't advise anyone to feel optimistic that future U.S. releases will use Toho 4K masters. For one thing, of the Heisei Series films, only Biollante has received that treatment so far. Furthermore, King Kong vs. Godzilla is the only one of approx. a dozen Toho films with a 4K master released on disc anywhere in the world. Toho has been remastering their films but either limiting them to theatrical and satellite TV avenues or sitting on them altogether.