So I put together a chart of all the current largos: http://i.imgur.com/cSfn8r1.jpg
Following a look at the lot of them, I have a couple of suggestions. :o
1) Some largos are inconsistently designed
There are a couple of slimes with special faces: Tabby, Hunter, Rad and Quantum. In most largo combinations involving them, the special face overrides the other faces, giving kitty faces, glowy eyes, or, when they combine, glowy eyed kitty faced slimes. Quantum Tabby and Quantum Hunter are odd exceptions, although the Quantum Hunter evidently has a model using the special face. Additionally, Rad Tabby is the only Rad largo without glowing eyes. Both of these seem like oversights and I'd love to see them brought in line.
2) Color/Trait Hierarchy
Slimes have a somewhat arbitrary ranking in which traits they pass on when mixed. I'm certain the ranking was deliberate as each largo seems handmade. The order, going by the naming convention, seems to be Pink > Quantum > Honey > Boom > Phosphor > Rad > Rock > Tabby > Hunter > Crystal. This is pretty neat on the whole, but a couple of things:
2a) Crystal combinations never use the Crystal's lovely color :(
Maybe the idea was that Crystal slimes are like prisms and take on other slimes colors and that's why the only thing a Crystal can ever pass on to a largo is its spikes, but the spikes look horrendously mismatched on a lot of other slime colors and other slime traits would probably look quite lovely on a purple slime.
Hunter Crystal looks especially off, and the Hunter slime's camouflage would, in my opinion, make it an obvious candidate to take on that lovely dark purple hue. Other slimes that might look better with Crystal as a prefix would be Tabby, Honey, and Rad (Crystal Rad would look especially handsome, in my opinion).
2b) Some Phosphor combinations look off
Pink, Quantum, Honey, and Boom combinations with Phosphors override the Phosphor's colors. I'd like to see an update to these four (or at least Pink and Boom) that lightens the wings and maybe the antennae compared to the body.
On regular Phosphors the lighter color makes those parts seem thinner and it works very well for their design, but Pinks and Booms especially look like they got dipped in a bucket of texture with little care for the result. Quantum Phosphor has neat little swirlies varying its look and Honey Phosphor has enough transparency to look fine, but they could probably still be improved.
3) What if it base slime mattered?
This one might be a stretch, but what if the base slime mattered when you make a largo? This would double the amount of largos in the game without needing to add more slime types or habitats and vastly increase the amount of experimentation available to a player. It would also allow for some visual variance in pens without having to feed the slimes by hand.
With how the color/trait hierarchy currently is, it wouldn't be too hard to come up with appearances for them, either. Pink (the one slime who's only trait is its color) combinations could remove traits/special faces but keep color, eg a Honey slime with less drippiness and no face plate, a Quantum slime with non-glowy eyes and no spiral, etc.
Regardless of whether or not any of these suggestions are taken in, I really like largos and the whole game. <3 Quantum Rad especially is a beautiful mix of the base slimes.