Riot should create more effects like Adaptive Helm instead of tacking on magic shields.

The problem with mid and its diversity currently lies in how little MR and MR items do to deter DPS. You know how Randuin's hurts crit, and Tabi hurts all autos? Until Adaptive Helm, there were no equivalents for dealing with magic DPS. So Riot just took MR and hp and tacked it on an item, and while that was good for dealing with burst mages, once you got to the late-game there was no way to deal with a shredder like Karthus or Orianna (way back when).

Instead, Riot would give spellshields and magic shields like those of Hexdrinker. The problem is they hurt traditional burst champions much more than DPS champions because the DPS champions will have another Q up in 3 seconds anyways, while burst champions like Annie or Lux would be stopped dead in their tracks because of their relatively high CDs. That, combined with the overall decrease in AP and increase in CDR has made it very hard to meaningfully burst down champions. This, of course, made it much more reliable and much more preferable to just run whichever DPS mid is better. The meta for the past year or two has been Syndra, Orianna, Cassiopeia, Viktor, Taliyah, etc. with the occasional Fizz, Ekko, and Ahri.

And so while mid mages aren't necessarily struggling, it sucks. Instead of traditional Ahri builds, that maximize burst AP, the most efficient builds max E second and get Liandry's after Morello for damage. You essentially relegate yourself to Syndra's role of catching someone out and then you have some mediocre damage afterward, certainly not enough to kill through all the shielding/CC negation they get. I just top off on CDR and hope my team will be able to make use of the extra second to chain CC more easily before Mikael's or QSS are purchased. As for playing Lux? I don't even touch her anymore since she feels downright impossible to have an impact on outside of enabling your carry.

And it's not to say that you can't win games with these champions, or that you can't carry. It's just that they don't feel good when they're played because of how much extra work goes into using them. I'd much rather play TF/Ori and spam utility so my Trist gets fed than even attempt to be the real carry.

I think the solution that would open more champions up to going mid is to focus on getting more effects like Adaptive Helm, that work particularly well against abilities that are going to cast on you repeatedly, rather than another mastery like Nullifying Orb. By having more anti-DPS options in the game rather than ones that are anti-burst, we can start knocking down the Syndra/Taliyah/Corki meta, and replacing it with something a little more diverse.

P.S. another issue I would work on is giving APs (particularly bursters) more anti-AD options. When you're mid-game oriented and they buy the perfect anti-AP item to counter you so you chill to the late-game and your anti-AD item is significantly worse for dealing with them in the late-game it feels pretty bad.