So essentially Melee 3.0 is removing every useful application of melee and leaving us with a situation where the tip-top of melee meta is back to the handful of half-decent stances. Expect to see the return of Broken Bull heavy blades and Volt speed-spam.
See, here's the thing: melee is, in abstract, far less viable than gunplay or ability spam. In low level content it means having to run up to every single enemy that you can one-shot. In high level content it means having to get close to things that can insta-down you, constantly. The melee meta around slide-spinning with memeing strike in conjunction with using Blood Rush + Berserker oriented around mobility more than anything else. You can still expect to see slide-spinning after Melee 3.0 if they don't completely destroy it for that simple reason: most melee in Warframe, a game built around an extreme degree of player mobility, is highly immobile. In high level content, standing still means dying. Most melee forces you to stand still. Allowing dodge-cancels out of attack animations won't resolve that.
They're removing the timed attacks from stances, giving them all the same simple combos. The only differences between them will be the hitboxes, which should make them all similarly viable in different situations.
Regarding mobility, they are making changes on that front as well. Firstly, blocking will be highly effective on all weapons, both mitigating damage and also building the combo gauge as you get shot. Then you can chain from a block directly into an initiator, that leaps forward while attacking, allowing you to more effectively engage distant enemies. And if you really need to be moving around, you can now do slam-attacks diagonally, accurately smashing down on groups of distant enemies. So there will definitely be more options for getting around.
About channeling, their current plan is to replace it with what is essentially a super saiyajin mode that has it's own resource bar which gets filled from melee combat. So once you're charged up you can set yourself on fire and do bonus damage on every attack for a while. Though this option is still in the early design phase and subject to change.
They mentioned that all weapons will have their damage adjusted with consideration to range and speed. So long glaives will hit far and fast, clearing trash easily but not being super effective against meatshields. Greatswords will hit far and hard, but slowly so their overall dps is only modest. Daggers will have small range, but hit fast and hard for better sustained dps. The fact that they are taking these things into consideration makes me optimistic that we will end up with weapons that are balanced and situationally useful.