Easy Way to Improve Tank Itemization.
TLDR; make Giant's Belt build out of two Ruby Crystals.
Defensive items are a reactive purchase, you buy what you need based on the strengths of the enemy team. This can be incredibly punishing to tanks however, as building one form of mitigation will leave you incredibly vulnerable to the opposite type of damage. At the time a tank considers their first item, it may not yet be apparent who will be the greatest threat on the enemy team.
The response to this is often "build health" but here lies the real issue. For almost all defensive items, building a Giant's Belt locks you into building armor and a Ruby Crystal locks you into building magic resist (I think ohmwrecker is the only exception to this).
The solution, make Giant's Belt build out of two Ruby Crystals (plus a combine cost of 200 gold, keeping the total price the same). Now a tank can build two or three Ruby Crystals and build them into whichever defensive items make since for the game. This would allow tanks to delay the choice of which form of mitigation they want to build first until the game has progressed a bit longer, making the decision better informed and less punishing. Same logic as the recent change to Cloth armor and Chain Vest. (Buzzword: meaningful choices)
Obviously this won't fix everything, but it's such an easy buff to tanks, without increasing their power level, that I struggle to find a good reason not to do it.
Thank you for taking the time to read my post.