I'm not surprised we don't have a grot HQ, or anything since forever really. Outside of the grot tanks and mega grot tank in FW anyway. And the arachnarok for fantasy. My point is, people here have gotten ahead of themselves in regards to grots; from the writers of every codex since 3e, grots are little more than punching bags to be abused by orks.
There is just not a lot of interest in grots, and I can understand that because they're cowardly, stupid and weak. They aren't... fierce... in the traditional sense. And while they are cunning, they aren't portrayed as cruelly inventive like the skaven are, which is the direction wimpy little hordes need to take to be popular.
There is another fact to this problem too, and it's that expanding the grot roster inherently breaks the mold of what they want orks to function like. You would end up with greenskin soup, which I'm fine with, but might not be what the "designers" intend.
Even I don't find rebel grots incredibly compelling, and I've been collecting gobbos since the 90's when I started playing MTG as a kid. In fact, my love of goblins and greenskins is what drew my attention to 40k in the first place, so I feel like if I'm not enamored with them, there probably isn't a whole of of people who are. And I mean that as a whole army. Not as just an interesting story or a little side project unit or something.
If they ever decide that grots are more than fodder, they'll need to break from the rebel grot theme. Rebel grots is the only theme I think they've invented for grots, and that was for gorkamorka a million years ago. If they want to expand on grots, they'd need to just add them to existing clans or unique grot clans. Rebel grots are after all, thematically mutually exclusive with regular orks. And I don't want to hear some long dim tirade of how they aren't, or could; the raison d'etre is liberation from ork oppression. The whole theme is Bolshevik revolt. Red stars, banners etc etc. That's the theme! Bringing grot rebels and a squad of nobs together is like mixing space wolfs and thousand sons.
Grots will need to function coherently within the context of the ork mother-codex, they'll need to retconn recent lore depicting them as nearly useless, and they'll need to write them in such a way as to be compelling enough to be worth the resources to make model moulds for them.
Maybe they could start with something simple. Say a grot sniper unit. I'd like them as 2 models on a largish base, spotter and shooter/loader, with a very long rifle. Put the model out with a downloadable datacard, like they would with an AoS model.