I don't think Galio will ever be viable unless they make him overpowered

Galio's kit is overall just a big mess. He suffers a lot from Garen's syndrome in that, yes, he is impossible to kill by AP champions, but Galio will do literally NOTHING to stop Leblanc from killing your ADC, or Xerath from poking your team down. You can shield someone and that's it. Your Q and E sucks, and you can't ult unless you have flash.

Galio is like the AP counterpart of Rammus... except Rammus also has some built in magic resist increase and have significantly more mobility than Galio. Most AP champions have very long range, and Galio can do nothing to get to the back line and lock them down. And getting too close could result in death, because if you build him the most optimal way, then AD carries and AD bruisers can easily rip him a new one since his armor is horrendous.

And to add salt on the wounds, AP champions can easily stack magic penetration with void staff + sorc shoes + haunting guise and deal with you pretty nicely. MR can only do so much for you and there's not a single item in the game that serves as the AP counterpart of thorn mail.