So some ideas i guess (and in spectacular poor english as well) :
Tauros crest: Summons gen 1 field or whatever it's called (it works as desolate land were when he is in the field its activated and it imeadiatly disappears when he exits) as well as a slight power buff- as one of the main heads of gen 1 this guy has really fall of since then, but i think this would be really interesting for him as gives him a really spammable move in hyper beam as well as a usable special attack, this would also help it a lott in doubles as he would enable completely different stratagies.
Slaking crest: Truant only activates when using a attacking move- this way it would be much harder to predict around as well as making it possible for it to setup if badly predicted, but not smiply sweep as slaking is still very prone to simply being protected on as well as not being the fastest thing and still having no resists making it hard to switchin without abusing shift mode.
Sceptile crest: Reverses offencive resists by weaknesses and vice-versa, also unburden activates if under 33% health- i know you're probably not gonna give crests to pokemon that already have megas but it always bugged how sceptile was a offensive mon in a typing with a ton of offensive resists and almost no offensive weaknesses so i thought if you inverted it it would already be a massive buff.
Falinks crest: If under 33% doubles no retreat stat changes- this mon has such an interesting move, however it generally will have to take a hit and even if it survives after a buff its still not fast enough to outspeed a lot of targets and any priority finishes it, a complete double in stats would make it a really scary mon if it manges to set up.
Wigglytuff crest: The lower the hp the higher the speed, the attack and the sp.attack- This mon is normally atrocious its slow weak and even frail, the only was to buff this is abusing its weird bulk to make that the lower the hp the higher ist everything else, i think it should cap at 200% when under 25%
Now i had this idea for a passive crest; like tha one for Silvally, were when activated any pre-evolved mon would gain the characteritics of the evolved forms, like stats and movepool and even typing and of course would make eviolite have no effect, they would also have to had pass trough the needed process of evolution even if they didnt evolve , this way anyone could use the pre-evolved form if they prefered it.
Also could you buff slightly magcargo's crest i think its interesting but it's still to weak and not fast enough, and with that bulk its never getting a shell smash off i think if you just gave it a 20% buff instead of 10 it would just be enough, or even giving it sturdy
