Materia itself could stand to be a lot more diverse....
And I could see that happening in 2.0

1. Being able to attach materia in a persons bazaar
Which opens up the possibility of....
2. U/U materia... NMs could drop super epic U/U materia...making it so you'd need an NM drop coupled with a crafted item... everyone's happy
3. Materia that is restricted to a single piece of gear instead of a single slot.
(i.e. a materia that can only go on Garuda's Bow. So nothing would be unmeldable, you just wouldn't be able to put normal materia on Primal weapons, and so on)
This would work well with your idea....

You do some super hard, super epic quest... and you get a crazy strong materia that can only be attached to the Gallant Surcoat that adds +40 def, +40 Vit, +25 Block, +120 HP, +40 Enmity
Now the gallant surcoat is better than the Heavy Darklight stuff... which would be fine as long as that Materia is insanely difficult to obtain. Some long term goals SE...
Like you'd have to turn in key items (With low drop rates) from all three strong hold bosses + all three dungeon bosses, and that unlocks a super hard HNM that drops the u/u gallant surcoat materia

I want 2.0 to be much more of an experience....
Like if you start playing a year after 2.0, I don't want there to be content and gear that you can just skip because it isn't relevant anymore... the basic stuff should be an informal pre-req to the advanced stuff
Think about it, if at 2.0 there was a materia that enhanced the Ifrit Battleaxe, making it still the best Axe, then people can't just jump in and go do that new dungeon to get the Materia.. they'd want to go do Ifrit first to even get the axe

Normally people do old content to get the pretty good gear to even challenge the new content... but SE has made it so you can challenge and beat any content with even mediocre gear and not really notice a difference