Personally, I think gameplay that encourages one to be hit should be at the core of any tank playstyle, including Galio's. If we are to define tanks as prime damage-soakers for the team, then it only makes sense for tanking damage to be a major component to their gameplay. As it stands, though, that sort of gameplay hasn't been implemented at all, which imo is one of the reasons why so many tanks feel like fighters, and vice versa.
In Galio's case, one of the things I liked about his old self is that it really felt like he powered himself through enemy magic, even if it wasn't implemented in the best way. Currently, he just has an anti-magic Malphite shield, which isn't really the most gameplay-laden way of implementing that kind of fantasy. Personally, I'd change two things to make Galio feel like he's a magic eater:
- Change his damage reduction, so that while the effect is active, it converts a percentage of damage taken to grey health instead, and the target instantly heals for all of their grey health at the end of its duration. There are a bunch of reasons I think this would improve him: first off, it would add more counterplay by enabling a clearer window where he's vulnerable to burst, but on top of that it would also add much more clarity to how much damage Galio is actually mitigating, as damage reduction doesn't really give clear feedback. Additionally, this would be a nice callback to Bulwark and its healing, while also delivering much more on the magic eater theme by having Galio heal himself (or his ally) using the power of his enemy's spells.
- Have Galio empower either his next Q or innate based on the damage he soaked up: the idea here is that Galio should likely have a little less offensive power as a baseline, but should be actively seeking out damage, especially damage from spells, in order to retaliate with even greater strength. Lowering the base power of his Q, for example, but having it generate harsher tornadoes based on the damage he took, could create a much more interesting back-and-forth in lane, especially if the damage taken were converted into a resource he'd consume on his next spell. On top of that, it would also feed into the magic-eating theme by tying more of Galio's power to soaking up magic.
To me, Galio's core playstyle was always one of retaliation. He wasn't necessarily the first to throw a punch, but when the time was right, he'd swoop in and punish the enemy for attacking him or his friends. This is, unfortunately, something I think is largely missing from Galio's current playstyle, with the exception of his ult, as he's encouraged to just dive in and ignore most incoming threats as he deals damage. This, imo, is also at the core of what makes him feel like a diver, rather than a true tank, because the general playstyle of fighters is to go in, try to kill someone, and also try to shrug off as much damage as possible in the process. As a champion who was always part battle mage, it makes sense for Galio to deal damage and kill opponents, but despite being a Warden, and a character designed to drink up enemy magic, he doesn't really want to get hit, so much as has tools to make getting hit less impactful. Rectifying that with changes that would not only fit his magic-eater theme, but also establish more of a retaliatory playstyle overall (i.e. less upfront power, more power after the enemy's struck), could benefit both his gameplay and his theme.