Not really gonna have the impact you are looking for I'm afraid. Force of nature alone won't do it, even back then it gave what 70 MR? Compare this to armor itemization that has both Thornmail and Frozen Heart giving 100 armor a piece, Randuins giving strong amounts of both health and armor, not to mention several damage items like Black Cleaver, Triforce, and Frozen Mallet that give HP on top of damage.
Now compare this to MR itemization. Spirit Visage and Banshee's Veil both are only giving 50 MR, and Locket will give 40 between the 20 you get and the 20 MR aura. Damage wise there is Maw, Abyssal, and Athenes.
Simply put the entire MR itemization of this game is based around the idea that an AP champion will deal most of their damage and then everything goes on CD so you just need enough MR and HP to survive those CDs and kill them before they come back. Cassio completely throws this entire set up on it's head and in order to account for it you will need far more the Force of Nature to return, you will need a complete over hall of Magic Resist itemization. There is a big problem with that though. If you design Magic Resist itemization around a mage like Cassio who does a ton of constant damage, those that actually have CDs to work around are going to be screwed because they can't keep up. It's kind of a catch 22 where Riot either accepts Cassio for what she is and designs MR around her existence in which case many other APs are going to be worthless or they simply nerf/rework Cassio to be within reason for how Magic Resist currently exists in the game.
As much as I love Cassio I fully expect the nerf path to be taken by riot as completely rebalancing items just for 1 champion in such a way that it would screw every other AP champion in the game over isn't reasonable.
I guess Cassio isn't completely alone for constant Magic damage mage, Azir, and Karthus both have really high amounts of constant magic damage but even so that is a very small amount of champions to expect riot to completely revamp magic resist for.