As someone who adores the idea of crests and making less viable Poke'mon a little bit more niche, I came up with a few crest ideas just for fun. I don't expect these to be added to the game in any way, but I think that some of the changes on the list could be really interesting. I tried to not make any crest too busted and keep them as simple as they can be, while playing into the Pokemon's strength.
Arbok Crest- Coil at the end of every turn. Biting attacks drain 25% HP.
I think that because Sandslash got a regional form, Arbok needs some love too. Free coils could make Arbok a really solid support Pokemon in Doubles as it already has access to Intimidate and Glare/Acid Spray.
Venomoth Crest - Poison Attacks always Lower enemies Sp. Attack by 1, Bug attacks lower attack by 1
I actually really love this crest because it pairs pretty well with tinted lens. It would allow Venomoth to do some decent damage and have a bit more utility.
Kingler Crest - First hit is multiscale (maybe 30% reduction instead of 50%) and gives a speed boost
Kingler would probably still drop to any electic attack that is thrown at it, but at least with this crest it would have an opporunity to get a sweep going somewhat.
Octillery Crest- Beam Moves have +2 crit chance, heal 1/8 hp each turn
I wanted to lean more into Octillery's sniper abillity rather than moody, as I feel moody is more of a gimick. In a way it is pretty similar to the Claydol crest.
Tropius Crest - Increase raw defenses by 1.3x and do damage based on how much HP Tropius has remaining. (at 100% HP damage is 1.5x scales down linearly to 1.0x at 1%)
Tropius needs some love. I think this Crest pairs well with Harvest, or even Chlorophyll. Either way, this would maybe encourage a few more players to sneak Tropius onto their sun teams.
Emboar Crest- Has effect of a held metronome, successive uses of moves grant speed boost that is lost when move is changed.
Samurott got a crest, and Serperior is actually good, so Emboar kind of got left in the dust. This crest could hopefully help him charge his way out of a slump.
Zebstrika Crest- all attacks use physical attack
We all know that physical electrical types are quite pitiful. Perhaps the solution is not to make more physical electric moves, but rather make all moves physical! This will make Zebstrika extrmely unqiue as it will be able to hit physical thunders and even spam physical overheat that does not lower it's attacking stat.
Elektross Crest- When switched in the opponents speed is lowered and Elektross speed is increased
For being a big scary eel, Elektross really underperforms. The ability to sap speed on entry I think could really swing some momentum into Elektross' favor.
Slurpuff Crest- Defense incease by 1.5x and Safeguard on Entry
Leans into a supportive roll. Any pokemon would benefit from defense boost, but here it allows Slurpuff to really excel in doubles, perhaps as a sticky web setter.
Salazzle Crest - Poison Type attacks are super effective against steel types
I know that Salazzle has fire type stab for steel types... but it's signiture abillity is called Corrosion for a reason. I think this is maybe the most niche, but also one of the coolest crests on this list.