Unpopular opinion here: I think Rabadon's should probably just be removed altogether, and the follow-up should be a) more AP on other items, b) price reductions to Amp Tome, and c) AP ratios on abilities increasing per rank.
As of now, Deathcap serves only one purpose, and it's to give more AP. Not only that, its percent amp passive means you only get major amounts of AP if you've already committed to building a lot of it already, which doesn't really help out supports who might want to boost their own power with a chunk of raw ability power. In an era where itemization has moved away from stat sticks, Rabadon's Deathcap is still the AP stat stick, with no gameplay, a niche that isn't particularly interesting ("more AP" is something you're already considering when purchasing AP items anyway), and a set of decisions towards buying it that is so restrictive that it's usually either a must-buy on mages, or not worth considering at all in most cases, at least not until very late into the game.
At a time when mage items are also losing a lot of their raw statistical power, Rabadon's is, in a way, a bit of an indirect nerf to mages: its existence means mages need to choose between utility and more raw AP, when the standard has shifted towards them being able to acquire both in one go. Eliminating it and shifting its power towards the rest of mage itemization and kits would render their power progression a lot more consistent, and allow them to get the utility they need late-game without having to skimp on damage. Scaling AP ratios would also allow AP-scaling supports to make much better use of the ability power they'd acquire through items, and giving mages a mix of fixed and scaling ratios would let them be powerful exactly when they need to be, without having to rely on artificially expensive Amp Tomes to keep their snowballing in check.