After a continuous four days worth of constant tweaking and testing, I believe I've finally created a norn base genome I'm happy with. At first I figured I'd just talk about all the changes I made in one post, but there's one change I feel is worth giving its own post: the changes to the digestive system, or why unedited 2017 creatures have so much muscle tissue all the time.
First, a brief explanation about how the creature digestive system works in C3/DS. There's three nutrients a creature can be hungry for: starch, protein, and fat. When a creature eats something that gives it one of these nutrients, it's then converted into a matching short-term energy source: glucose for starch, amino acid for protein, and triglyceride for fat. If the creature doesn't need those short-term energy sources for anything else, they're converted into a matching long-term energy source: glycogen for glucose, muscle tissue for amino acid, and adipose tissue for triglyceride. These can then be converted back into short-term energy should the creature need it (e.g. when it's having trouble finding food).
However, there's a problem in vanilla creatures with the reaction that generates muscle tissue. It's reliant on another chemical called anabolic steroid (yes, it's named after what you're probably thinking of), which is supposed to be generated by a chemical emitter that fires based on "Muscle energy used." Which is to say it almost never fires at all. So vanilla creatures don't generate anabolic steroid and thus can't build up muscle tissue. Fortunately, the rest of the digestive system has enough redundancies that this isn't crippling for them and is likely to go unnoticed by the average player.
Nevertheless, it's a flaw that's been corrected in both the 2017 and TWB/TCB genomes. Both genomes made use of the originally-unused chemical Activase (something you might recognize from C1 and C2) by tying it to the stimulus genes associated with movement (e.g. pushing, grabbing objects, and walking around).
TWBs/TCBs generate minute amounts of Activase from these stims, and it's consumed in a chemical reaction that generates both anabolic steroid and body heat (chemical 110; used for a lot of things in the system that gives them their name). However, they aren't entirely reliant on this reaction to get anabolic steroid. Their version of the anabolic steroid emitter fires when their muscle tissue drops too low alongside hunger for protein, so even if they don't generate enough activase from physical activity they can still build muscle tissue.
So instead of getting rid of the Activase from the Disappointment stim outright, I drastically reduced it. It's still not a perfect fix; the stim still fires a lot. But it's still less than default 2017 creatures, meaning that there are cases when they aren't generating Activase all the time and generally reaching an equilibrium of getting more hungry for starch and fat when doing things and getting more hungry for protein when not, and with much less risk of getting heart attacks when they're actively trying to do things.
I also thought about throwing this out the window and just copying the TWBs/TCBs' system because honestly they handled it a lot better, but I decided I wanted to maintain compatibility with default 2017 creatures as much as possible and thus working with the system 2017 creatures already had felt like the best option.
To summarize: Creatures get disappointed a lot; 2017 creatures get too much activity chemical from it by default, so they're buff all the time. But removing that lets them build up too much fat and makes them get heart attacks too easily, so the solution was to reduce the amount of activity chemical they get from being disappointed.