These are very generalized design rules. If you would like something more in-depth there is one below as a google slide.
No unrealistic colors
Make sure accessories stay clan accurate (You can check clan accessories in each clan's respective clan info or in the in-depth guidelines down below).
Keep eye-colors realistic (we allow purple eyes)
Pelt patterns can be pushed to the extreme
No other animal parts (horns, wings, etc.)
No unrealistic body parts (body parts made out of water, wood, etc)
Make sure that cat's hair isn't too long. For a better example please view the google slide below.
Keep any body parts realistic
Leaders are allowed to have a diamond or star shaped marking on their fur anywhere to indicate leadership but it isn't required
Leaders MUST have a diamond or star shape in their eye to indicate they are a leader. This diamond/star fades with each life lost.
These rules only apply to clan cats unless otherwise stated. These don't list every banned character name. Use common sense, if the cats don't know what it is, it likely isn't allowed.
Anything with Mallow (This applies to kittypets)
Anything with Snail (This applies to kittypets)
Anything unrealistic (unless in CastleClan and approved. E.G. Eclipseclaw)
Any artificial human food prefix or suffix (Marshmallowkit, Donutear, etc)
Any exotic animals (Panda, kamado, etc)
Anything with star, unless it's the suffix of a leader
Anything with moon
Darkforest
Sharing name prefixes with any other cat in the same clan
Having the same name as a clan (No Castle, Flower, Forest, Cactus in the name)
Double words for one prefix/suffix is not allowed (E.G. Redwoodspark, Dragonflypool, etc)
The google slide goes more in depth into what is or isn't allowed. Reading it is not required but if you're worried about a particular design gleaning through it could help.
Clickable link just incase the embed is not working: here.