If we had hard endgame Solo content, it would only benefit existing Group Content

Too many Legendaries are dex fillers. Giving them busted moves or Mega Forms only makes other Pokemon dex fillers.

We need new content where you use these currently-dex filler-Pokemon to succeed.

It should be specifically SOLO, simply because PoGo playerbase is too casual to introduce hard group content to. Super Mega Shields are a proof of this. Ideally it would offer unique cosmetics or costumed Pokemon as rewards, like "achievement trophies". Super strong rewards should come from Group Content, hard solo content should just offer a PURPOSE for those super strong rewards.

And no, GBL isn't it. At least as long as tanking gives the best rewards and getting a high rating offers the same rewards as being a noob.