The feature I’m looking is an isolation-mode navigator tool that requires minimal clicking and is fast to use. You’d summon it while holding down a hot key, point the mouse at a group and then use hot keys (or the mouse wheel) to go into and out of the group structure.
Isolation mode, as it is now, is essential but clunky. I spend an awful lot of time clicking into, around and back out of isolation mode. It’s enough to cause RSI. The group select tool is useful but doesn't replace isolation mode.
Mark, have you found that you can assign a hotkey to Isolate Selection command instead of double-clicking? Keyboard Shortcuts > Menu Commands > Other Object > Isolate Selected Object.
I use Ctrl+> (makes sense for me).
Unfortunately there is no way yet to assign a hotkey to return one level up (only small arrow to click) — but you can vote for it here: https://illustrator.uservoice.com/forums/333657-illustrator-feature-requests/suggestions/32403778-make-a-hotkey-to-go-one-level-up-in-isolation-mode
And if you direct-select an object inside a group, the hotkey brings you to the very bottom of the structure, which is handy.