ZDoom

So we're not getting a SEXYBILLBOARD flag after all? :P

Agreed. At the moment there is no way to add more here without making it break apart entirely.
It should also be noted that any kind of tilted sprite rendering will cause major sorting issues - which is why all those poorly handled submissions have become a problem.

While I am not against the suggestion by the OP, I think before anything more gets added to the sprite rendering code, a major cleanup is needed first to undo the mess of some very poorly-handled submissions in the past...

(those who know what I'm talking about, would know)

I wanted this feature for a while now. IIRC it was implemented in Doomsday engine before, and I wondered why it wasn't in GZDoom yet.

[quote="XLightningStormL"]Possibly to expand on this, add some sort of +DONOTUSEXYBILLBOARD, essentially to force only certain actors to use just good ol y axis (say corpses, ammo, weapons, etc)[/quote]
[wiki=Actor_flags#FORCEYBILLBOARD]That's already a thing[/wiki]. It also has a far better name than "DONOTUSEXYBILLBOARD".

Possibly to expand on this, add some sort of +DONOTUSEXYBILLBOARD, essentially to force only certain actors to use just good ol y axis (say corpses, ammo, weapons, etc)

My idea of semi-XY sprite billboarding is pretty much putting a maximum amount of pitch that a sprite can face. Because when you look down to see a monster facing straight into the ground with it's legs up in the air, it can look kinda dumb. See the billboard axis example on the ZDoom wiki here if you don't know what I'm saying so far: https://zdoom.org/wiki/File:BillboardAxes.png

I would make a 2D example of how this would work, but paint.net is being a pain right now.
TL;DR, there should be a minimum-maximum slider for the pitching on sprite billboarding going from 0 to 90 if that makes any sense.