Character Concepts you wished would work in D&D?

[Nagog]

I feel like this is already covered with the Cavalier Fighter (using one of your extra feats for PAM) or Conquest Paladin.

Don't get me wrong, I'm a big advocate for more complex Martial options, I'm just not sure if 'AOE Control' is enough of an unfilled niche to be the only basis for a new subclass.

The cavalier is interesting to be sure, but it's reliance on melee attacks for it's class features limits it's full potential. Reach weapons and a mount would definitely help things, but considering they have no way to acquire a mount without the DM just handing it to them, the build is limited to at max 5 enemies, if you have maxed Strength and use two weapon fighting and action surge to mark 5 enemies, and if they damage a target other than you (let's face it, you're in melee distance and just hit a ton of folks, so they're gonna target you), then you get to attack all those that fulfill these very specific conditions as a bonus action one time on your next turn. At 20th level, you may use your full attack action and bonus action to mark 5, and then if (heavy if) they all fulfill these requirements, you can get a total of 9 attacks next round, if you can manage to stay in melee range of 5 targets and none of them target you and none of them miss. A single Fireball will deal more damage in one round to these folks than your 2 rounds of attacks if they're that close to one another, and Spike Growth is far more effective at locking enemies down.
Another downfall of the Cavalier is it's reliance on PAM and Sentinel to really shine as a crowd controller. You even get part of Sentinel as a class feature, but not the parts you really want. PAM and Sentinel on any other build gives you about the same capabilities, and tacking that on a Storm Herald Barbarian lets you spread the damage out more than a Cavalier, without DM favor or heavy investment. If the Cavalier's marking and follow up attack were opened up to ranged attacks, I'd be far more interested.

Also I'd argue that Paladins aren't really a martial class. If I count them in this mix, Rangers would take the cake for area control martial by nature of casting Spike Growth or Ensnaring Strike or similar spells.