Of course, I don't expect you to actually implement these suggestions. Your taste is your taste.
1. I've noticed, that you replaced more than just enemies in Titty Doom. I have found at least two static objects, that are now sexy statues. That's good, I hope you continue with this. What I would like to see replaced most of all, are all the various mangled dudes, that decorate the regular Doom levels. Both the floor/ceiling objects and the wall textures. I don't mind horrific violence, not even in the context of this mod, yet somehow all those corpses really clash with the mod tonewise. I think there is a great opportunity to display even more sexy girls in peril, or worse. Maybe they could be zakos, that were punished for incompetence. The possibilities are endless. Crucified, hanged, beheaded, blindfolded and shot; in chains, in cages, impaled, bound to a whipping post, etc. Executed or just tortured and still alive. Whatever you like.
2. I also didn't like the tanks much, but for more practical reasons. The bulky sprites really block the player's view. And worst of all, the corpses are pretty much the same height. It takes just two dead Arachnotrons in a corridor and you have no idea anymore, what goes on behind them. If you intend to continue working on Zako Doom or you want tanks again in Titty Doom, then I suggest, that the tanks explode on death and leave behind some debris and perhaps some charred corpses.
3. I have a feeling, that you are going with more of a fantasy theme in Titty Doom. I think a snake lady might be a good replacement for the Arachnotron. With most of her body coiled up beneath her, it could give her the necessary squat appearance. Give her a staff, that is held horizontally, when not shooting and some horizontal horns to fill out the space on the sides a bit more. A slithering snakebody is probably also easier to animate than all those spider legs.
I realize, I've pretty much described the Ophidians from Heretic. So same as those, just sitting a bit lower and with a longer body, that is bunched up in a zig-zag pattern or coiled up.
Another possibility would be a slime girl. Her lower body would be just a blob. It doesn't get any easier than that!
4. Doom had really good monster design, in that they all looked very different even at a glance, in darkness and at a distance (With the exception of the Baron and Knight). This is very important for quickly assessing a situation in such a fast-paced game. I realize, that this is more difficult to pull off, when all the enemies are supposed to be sexy babes. You still pulled it off, though. Mostly. Both in Zako and Titty Doom, the Shotgunners and Chaingunners look very similar. I suggest to make the chaingunners a bit wider, like their original counterparts. More buxom and beefier weapons. For Titty Doom I suggest giving the Chaingunner a crossbow. That alone should help to create a very different silhouette.
5. Pain elementals might be another tricky enemy to replace. I've had this idea, that I think could be fun: Make it a pregnant lady, that literally gives birth to Lost Souls! The (exaggerated) pregnant belly + tucked up and spread legs would provide the necessary width for the sprite. Why does she float, though? No idea.
6. What else is still missing? I think the Mancubus and Archvile should be pretty straightforward.
Mancubus - A sorceress, that shoots fireballs from both her hands. Maybe with a big, flowing cape. Again, to fill out the wide hitbox.
Archvile - A necromancer, obviously.
Hmm, I can't think of anything good for the Revenant, Spider mastermind and Cyberdemon right now. And certainly not for the Lost Souls.
7. I want to second, what
DSB
has said: The gorier, the better. But I suspect, that extreme violence is probably not your cup of tea. I just wanted to mention it.
8. Might I please ask you to remove those cat sounds or use different ones :)? They are pretty jarring for some reason.