For me, interesting would be a set of trait which impact more than one thing at once.
Overtuned is cool because all the traits have a + and - baked in.
Tightly coupling a positive and a negative lets them give a powerful positive effect without it being too unbalanced. But other than overtuned and cybernetics (which are just regular, already balanced traits, with a minor penalty), all the current instances of this are kinda... terrible.
Aquatic is currently too weak to be 2 points. Noxious is just an awful trap unless you're going for a meme slavery build. Cave Dweller is just... bad, but like Lithoid, is fixed by allowing xenos into your empire, so it doesn't hurt that much. Inorganic Breath is godawful (it's functionally a negative trait, but costs 3 points for some reason). Exotic Metabolism can be useful, but I mainly use it to have a breeder species that gets converted (painfully, and micro-intensively) to a template without gas upkeep.
The core of the problem is that if you have a positive trait that's supposed to be worth two points with pure positive effects, but the effect is too weak (say, it's half as good as it's supposed to be), then it's overpriced by 1. But suppose you have a trait that's supposed to be worth 2 points, with a positive effect that's supposed to be worth 4 points being canceled out by a negative effect that's supposed to be worth -2. If the positive effect is also overestimated by double, and the negative is underestimated by half, then you can end up with a trait that's costs 2, but is actually worth -2.
Ex. Inorganic Breath. They took a 2 cost trait (+.01 gas per pop) and doubled its effect. "It's worth 4 points, or possibly even more, because you're getting two traits in one; so let's call it 5 points". And they added a negative: "Wasteful seems decent with +10% CG upkeep and -1 points. Let's add 5x Wasteful, and also add food upkeep that costs 1.5x as much as Wasteful, times 5, again. That's like, -2 points, right?" (I have no idea how any thought process produced +50% being reasonable). Then they did some very strange math, and concluded that the two stapled together should cost 3 points.
But it's 12x wasteful (should be maybe -5, since you're only giving up 1 trait pick) combined with a trait that's massively overvalued. People take the motes and crystals ones to cheese edicts at the very start of the game (Volatile Land Clearance for blockers and Crystalline Sensors for corvette exploring), but they're objectively non-effects, past that point. With 1000 pops in your empire, working in research labs that cost .33 gas/crystal/mote per specialist in the relevant building, that 2 cost trait gets you.... 10 of a resources, or enough for 30 researchers/bureaucrats. .01 gas is just totally useless, since you don't even have cheesable edicts at the start. So you end up with a trait that costs 3 points, which is realistically worth -3 or even -4.
The other way: Aquatic was originally envisioned as 0 cost trait ("extra habitability on a single world type, negative habitability on 6 types, and triple the housing penalty on those 6 ought to make it net 0, right?). It then got nerfed to 1 cost, and was still overpowered. So it got nerfed again to 2, and now it's too weak.