Morde's Ult is his Single Biggest Problem

The ability to turn a 5v5 into a 6v4 is strong under any circumstances. A burst mage with the ability to do so could be really terrifying, enough so to justify no CC, junk mobility and cruel early game health costs on abilities. So, reasonably, Riot decides that this ability should be on a tank, and not a burst mage. "But Riot, a tank with no CC and crap mobility is a worthless champion," the community points out, and thankfully Riot listens. That still leaves, however, the problem of a burst mage with "this is now a 6v4" as his ult.

Reworking Morde's ult would free up so much power budget we wouldn't be able to believe it. If he's just a health-casting burst mage with no CC, a shield that builds on damage and sweet waveclear, then he'll be able to be much better at that role. His damage has enough limitations that he'll be allowed to have more, and he might even get a better mobility spell than the trash version of ward hopping (which it might not work on wards, idk). With a new ult, a strong Morde can feel like the final boss even without a ghost.

What to replace it with? Personally, I'd be happy with a scaling passive that granted tenacity and slow resistance proportional to his shield percentage, and bonus AP when the shield is full. He's got enough buttons and low enough cooldowns that he can keep hitting them: he just needs a little kiting resistance and incentive to be attacked. Alternatively, something that works with his autos would be nice: he's got his Q, but otherwise his sweet attack animations with his whopping mace do poor damage.

Point is, trying to balance him around his ult compromises too much. Drop it.