So I have more or less completed another playthrough (both have been on Hard, I considered Nonsensical for the second one but ultimately decided that if it wasn't playtested, it probably didn't make too much sense), and I was curious to see how things would go if I tried to kill all the enemies and open all the chests while finishing each map in as few turns as possible, which would presumably require leveraging the lewd-promoting skills.
This did more or less happen, although it's a bit complicated. It's definitely front-loaded, as once you get Kobold Joe and level him up a bit, he and (untransformed) Carwyn can pull a lot of weight, and you can kind of leave the three stooges behind, xp-wise. The enemy targeting priority can also end up sending a lot of XP Gwen's way, and although she isn't nearly as strong as Carwyn/Joe, she can hold her own and get to a point where she doesn't need much help, either.
This is exacerbated by how promotion works, since it resets the xp gain rate and enables units to start snowballing pretty hard once they've gone up a tier. This is imo possibly the biggest design flaw in the game. Does SRPG studio have a concept of internal level?
Owain in particular suffers from all this, because he seems to have higher thresholds for relationship increase due to his earlier join time, but he also needs to run off and open all the treasure chests, which means he doesn't get to spend as much time near Gwen (or Carwyn, as applicable), and also loses out on XP. Mordrid can suffer a bit from this as well, due to his combination of low movement and being your only healer for most of the game, though mostly I just think he's a weak unit anyway, outside of healing.
Emrys seems like he's the one who's designed to keep functioning in spite of this, because his +Mov promotion and Swap skill allow him to, at a minimum, move Gwen around an extra space per turn, which isn't always relevant, but definitely can be.
However, story-wise, there's a reason to rout all the maps early on, because you get money for doing so, which is why I decided to impose that condition. But at some point you stop getting any money, and because most of the maps are escape, you don't really need anyone but Carwyn to keep moving forward (well, you could do it with Vivian, but it would take awhile to get her to a point of self-sufficiency). And Emrys has promoted for me around the same time in both playthroughs, which is after you stop accumulating money for kills, so story-wise there's still not much need to have him continue to interact with Gwen.
All of which is to say, I think this all works better in the first half of the game than in the second half. In addition to what I've already mentioned, some other reasons for this are that (imo) Joe is too strong/grows too quickly and too well, and units in general become too self-sufficient and thus don't need to leverage the adjacency interactions that drive the earlier parts of the game. In case it's not clear, that includes Carwyn's lone wolf, which he can get to the point of not needing to keep activated to be effective (whereas in the early game it's pretty essential to keep it on most of the time).
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Some ideas for tweaks:
I would rebalance Carwyn's progression to make him start out even stronger when he goes off on his own, but then flatten his growth curve so that transforming him is all the more tempting. The transformed form may need to be tweaked to accomplish this; I'm not sure what the modifiers are, but if they could be displayed before you commit to it, that would probably help anyway. The only chapter where I really felt I needed to transform him was the one with the Kobold longbows on elevated terrain, just because I needed more effective ranged options if I wanted to rout the map efficiently (though if I hadn't been trying to rout, I could have just ignored them).
Meanwhile, in the earliest parts of the game, I felt like he needed to use healing items more often than I'd have liked. I think he should have a self-healing skill like Gwen's, except it should only activate on low HP and when Lone Wolf is active.
I know there are good story reasons for Owain to be the lockpicker, but I actually think mechanically it should be Carwyn, the way Marth is in his games. Carwyn is already running off and doing his own thing, and making him be the one to open chests would depress his XP gain rate, as the main group would be more responsible for carving the primary path to the objective/fighting more of the enemies.
I think enemies should rush you more often, perhaps in a way that's tied to the difficulty setting. I was intentionally playing fast in order to push the lewd mechanics, but someone who's playing slow can probably avoid that by just creeping through the map carefully. I know that does lose out on speed rewards on some maps (and I think that should be made more prominent to promote fast play), but I think that (at least on higher difficulties) it should be more directly difficult to even make it through a map without lewd interactions. And enemies rushing you would make it harder to choose your engagements in a way that plays around the interactions.
EDIT: I also think Gwen's supportive skill should buff her when she's adjacent to someone and/or debuff her when she's not. That way the early game incentive for her to form up is even stronger, and later on it's harder for her to become self-sufficient. Also I think it would make narrative sense given her preferred way of fighting.
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I said I more or less completed another playthrough, and that's because I think I broke the game in the epilogue.
I got the ending where you have to fight Owain & Gwen - which is incidentally a bit weird in itself, as going into the bandit chapter, I had Owain, Emrys, and Mordrid at relationship levels 2/4/3 respectively, although Owain might have had the most affection. Then I put Owain with Gwen on the far side of the river, the king put them in a room together, and Gwen's characterization started to get pretty inconsistent. She and Carwyn had already had their first scene together, but then suddenly during the exploration stage she was having sex with Owain (never mind that she hadn't actually done that in front of the king and had said she didn't want to), then after that she actually had sex with Owain in terms of a story scene, and she started turning on Carwyn. But after the final battle, she said she was going to go with Carwyn and any ending where she got to be with him was a happy ending... except then during the epilogue she was working for Cythraul and was with Owain again.
Owain and Gwen told Carwyn that he couldn't beat them, and I can understand where they were coming from with their 30s in all stats, but... once I got them split up so that Owain didn't benefit from Gwen's skills (easy b/c they have different movement values), Carwyn was able to fight them at an advantage, and although he might have lost if they'd just traded blows, he scored a crit on one of them and I didn't even need to heal him before finishing the other. And then... nothing. There seems to be no way to progress the game from this state. Do I just have to lose? Oh, and Carwyn tells Gwen to forget about Cythraul and Emrys (rather than Owain).
EDIT: I tried to lose and he's very determined to win, but I finally managed to get him to lose and... I just get a game over.
EDIT 2: Okay, apparently you need to flee to the bottom. Oops.