This suggestion is something of a niche item that would primarily be good on champions like Katarina, gangplank, and perhaps kassadin (and more in the future, ofc). It would go a long way to making phys kat viable over AP and I know devs are trying to do that - one of the reasons AP kat is so good right now is the abyssal scepter + magic pen runes negating base MR vs death lotus. I realize there are higher priority things to fix, but I think that this would take very minimal work to implement and would be a small improvement to the game.
It would basically be a phys damage version of the abyssal scepter - not too much pure damage as that'd be a little too good compared to the scepter (or +phys damage items), but enough damage to be strong and a little magic resistance which is useful but not the primary point of the item.
2670 (as Abyssal Scepter, the itemization follows the implied prices and balances out at something like 2480 compared to the abyssal scepter's 2500, but the passive adds the extra cost (and it's cheap, but so is the passive on the abyssal scepter and many other items))
+30 magic resistance
UNIQUE: Aura: Reduces nearby enemies Magic Resistance by 20
BF Sword (1850) + null-magic mantle (400) + combine cost (420).