It's been a while since I've played any NG (mod or otherwise), and I don't have a ton of specifics on my mind at the moment, but I'm quietly waiting in the wings to provide my infamously obnoxious testing when new releases come out, so I can ramble at you for a bit here. The last version I played for any length of time was 2.5.1, which came out over four months ago, so please keep in mind my time away. It's entirely possible that all of my thoughts here are presently outdated.
The first thing I'd like to do is to request you continue providing detailed change logs with every new release. NGM has gotten quite large, so this is very important for knowing where to focus my efforts when testing.
Regarding balance: Invitation is easily my biggest problem with this game. Sex moves are quite strong by themselves, and they apply a variety of negative effects beginning as soon as the position is established, so suffering one even for a short duration will do a sizeable chunk of damage to your Stamina/Arousal/condition in general. It's fine to have powerful moves, but there's no counterplay to Invitation: your opponent uses it and you're just locked in: that's that. Because sex moves/positions tend to be further debilitating in themselves (massive Stamina damage, mind control, Charm, unbreakable hand/leg locks, awkward disengaging even if you struggle to a dominant position, etc), you can frequently be 100-0'd with little to no interactivity, which is not ideal from game design or fun standpoints. I'd like to see more ways to counter or disengage from these oppressive positions/skills.
Invitation immediately comes to mind as the most egregious offender here, but I imagine there are others. You could tweak that skill in a wide variety of ways to bandage it, but that's probably treating only the symptoms. I'd need to get back into the groove of the game and do more extensive research to say much of use, but that's a starting point, if you're interested. Also, I know 2.6.0 introduced stances that should have an effect on the potency of sex moves; I haven't tested that version enough to comment on it.
Fetishes, addictions, and Mara's collar: These are some interesting mechanics that promote longer-term consequences to the otherwise in-a-vacuum fights. I think they're a great idea, but also that all of them are strongly overtuned right now, to the point where you might as well reload the game if you get slapped with them early on, because the rest of the night will be spent licking feet or being shocked into submission. I'm all for some good, old-fashioned domination, but only having one button for the next five battles does become tiresome, so some counterplay to these things would be nice.
I don't have the presence of mind or data for a detailed breakdown right now, but perhaps fetish severity could be reduced upon orgasm, and the characters could masturbate outside of combat to get a grip on their fetish before the next encounter, at the risk of being ambushed while occupied. Also vaguely on my mind: Imbue Fetish should let you choose which fetish you apply, or at least not apply cock fetish if you don't have a cock, for the benefit of female PCs and any future NPCs that might have the Fetish attribute and lack a penis.
For the collar, I think the major problem with it is that it doesn't require batteries, so fixing that bug will probably alleviate its troubles.
Addictions are, frankly, not a mechanic I'm well-versed in. They've always been so overwhelming and frequently buggy that I never really got around to experimenting with them; I would always just do the quick and dirty removal and be on my way. There's also probably a few words to be had about how easy it is to dispense of unwanted addictions, which itself ties into the large amounts of money you can amass and what kind of philosophy you desire for opting out of content.
Attractiveness: A couple months ago I was actually writing something up about this, because it's weird and inconsistent, and I hate it, but I lost the text somewhere, so I'll just sum it up. Attractiveness varies wildly from character to character and so temptation-related skills are typically either completely worthless (dealing 7 damage to a level 80 opponent) or extremely strong (Reyka maxing out an empty Arousal bar with two uses of Tempt because she has 18 Attractiveness). The PC tends toward the former, making temptation builds unviable for PCs.
Clothing: We have a lot of neat clothing items, but the mechanic itself is rather pointless because of how easy it is to discard them. In some cases it's even a better idea to do away with your clothing as soon as possible, which seems strange and wrong to me. I feel like clothing should play a more significant part in the game; altering the game to fit this idea will probably mean adjusting most/all of the full-strip skills.
Pets: Um... I don't remember. I think the pets made positions confusing a lot, but it's been too long, since I don't think I've done a pet build for Angel this year. Needs more research.
A clear and concise damage formula for consistently predictable outcomes: We do not currently have this, but it would be great for QoL and testing. I touched on this and some other bullshit you may or may not be interested in here:
https://forum.fenoxo.com/threads/nightgames-mod-v2-5-1-2-updated-2-11-17.65/page-56#post-120592