Not sure how many are these are already done, but here are some crests that i would like to see (mainly weaker mons that don't get played otherwise). Keep in mind that many of these may be either very over/undertuned:
-Arbok: give it dark typing and strong jaw
-Avalugg: spdef scales with def
-Basculin: weird one, but give it fell stinger but as an ability (+3 atk if it ko's) [its a piranha or something idk] and 1.5x speed
- Beartic: a combination of snow warning and slush rush, with 1.2x atk
-bellossom: a chlorophyll bellossom holding this also gets drought and a fire immunity
-bibarel: simple bibarel holding it can't have its stats lowered, also stat increases are increased by an extra stage
-cacturne: has spiky shield on switch in and 1.2x atk
-carbink: it gets diancie's stats
-chatot: using the same sound move consecutively increases the damage by 1.2x, also its speed is increased by 1.5x. Also soundproof
-chimeco: your team is immune to sleep status, and its def, spdef and spatk go up by 1.5x
-crustle: gains a shell smash on switch in and its spdef scales with its def
-electrode: self-destruct moves gain electric typing and for one time during the battle only, they don't actually knock out the user
-floatzel: all moves get priority in rain, and its atk is increased by 1.2x
-kecleon: he puts you in jail for theft. Also, if using a damaging move protean changes kecleon (and the move) to a random supper effective type (or neutral if not applicable)
-purugly: atk scales with speed and its def is doubled (i mean its a fat cat)
-shiinotic: double damage against sleeping enemies, gets natural cure, and all stats other than atk and speed go up by 1.5x
-ditto: if applicable, transforms into mega form of opposing pokemon with impostor/transform, even if they aren't mega themselves. If it has no mega but a crest instead, then it transforms into that. For pokemon with multiple megas/crest forms, the choice is done at random
-lurantis: any stat increases increase all other stats by the same amount
-drampa: berserk triggers off of any damage, also gives its moves priority for the next turn after its activated
-sudowoodo: gets an ability like sap sipper that works for water moves too AND disguise, and atk up by 1.5x (this is probably too much tbh but eh)
-greedent: consuming a berry increases atk, def and spef by one stage each, and gets its berries back 2 turns after usage
This is all for now. Crests are great and should be in the main series games
