Ok, But Lets Look at Galio Conceptually

Galio as a character was meant to be a Tank that is also a Mage. Originally, he obtained this by gaining AP from MR, basically making him a Anti-Caster Caster. However, let's pause there, because we already have a character that is going to be really powerful, at least on paper. A Tank that can also be considered a Mage damage-wise is extremely powerful and that is what Galio's niche has been since before his rework and into his rework. So, now that Riot has actually changed Galio to want to build primarily offensive again, we have basically what old Galio is but updated to be modern. So, what do we got?

A REVERSE TANK EKKO, EXCEPT HE IS DESIGNED TO BE THIS WAY!

What I'm saying is maybe Galio shouldn't be what he was pre-rework, even if it worked back then. Pre-rework, many people didn't realize that Galio actually had an extremely high win rate (averaging around 54% for his player base). However, he was hardly ever played, so it wasn't a big deal. However, not that he has been updated, it's clear that this design isn't something that probably should be supported, at least if we want Galio to continue being considered a Tank. AP Bruiser would be a better description for what Galio is now.

TL;DR Maybe what Galio's current design space is, being an AP Tank, isn't what he should be pushed towards. Instead, maybe he should be given a push in a different direction.

What I would suggest is give Galio an anti-magic damage niche without relying on AP. The Rammus of Magic Resist.