Single frequency vlf ib detectors are very close to the end of the road.
What a detector like the F75 or T2 does well, it does bery well indeed.
What it doesn't do well, like work in salt or highly mineralized ground at depth, is the "job to be done" of new technology detectors.
Multi frequency (simultaneously) detectors like the CZ series and Minelab's barious platforms do,better with mineralization, but have other drawbacks.
PI detectors punch deep but do not meaningfully discriminate.
Minelab has a new, non PI platform in the GPZ, but no discrimination (yet), Whites has hybrid patents, but no hardware shown yet, First Texas has announced that they are working on new multifrequency platforms.
Until one of these leads, or something "out of left field" pays off, we are essentially stuck with "variations on a theme"
I look for lighter, weatherproff power efficient successors to the F75 and T2, while we wait for really new stuff.