If I were to add a few crest, here are the ones I would add: Luxray crest: Adds a Dark type to Luxray, boosts its STABed attacks by 10%. Toxapex crest: Swaps its attacks with its defenses (Only a "stat change", yes, but it could change the pokemon's gameplay). Heracross crest: adds the flying type to Heracross. Sandslash crest: when Sandslash enters the battlefield, summon a sanstorm for 5 turns. Grumpig crest: adds the abilty Simple to Grumpig (on top of its other ability). Delcatty crest: Delcatty's attacks all hit as "normal effectiveness" (Not "super effective", not "not very effective"), even on pokemon immune to it (as well as shedinja). Delhmise crest: +20 Def ; +30 Sp.def ; -50 Atk ; Adds the steel type to Delhmise. Starmie crest: All of Starmie's attacks use its Sp.Atk and hit on the enemy's Def stat (like Psyshock) (Only relevant if flip turn gets added in the next update, I haven't followed enough to see if it will). Tropius crest: Under sunshine, increase the effects of Solar Power (2x Sp.Atk, but takes away 1/6 of its health insead of 1/8).
Dunsparce crest cause this pokemon really sucks: Dunsparce becomes Flying/Ground, adds 10 to all of its base stats except speed, adds 40 in speed. Everytime Dunsparce uses the same attack in a row, its power is multiplied by 1.25 (a la metronome).
Electivire crest: Magmortar's Sp.Atk is multiplied by 2 if both pokemon are on the field. Magmortar resists Electric-type moves. Magmortar Crest: Electivire's Atk is multiplied by 2 if both pokemon are on the field. Electivire resists Fire-type moves. Zangoose crest: Seviper's Speed and Sp.Atk are multiplied by 1.5, but Zangoose loses 1/8 of its HP at the end of each turn. If Seviper is KOed, the effects end, then Zangoose's Atk is multiplied by 1.5 until KOed or switched out. Seviper crest: Zangoose's Speed and Atk are multiplied by 1.5, but Seviper loses 1/8 of its HP at the end of each turn. If Zangoose is KOed, the effects end, then Seviper's Sp. Atk is multiplied by 1.5 until KOed or switched out.
Unown crest: Randomize Unown's base stats before each fight, between 1 and 150 for every stat. Unown's hidden power now makes it use a random move (like metronome) instead of its normal effect.