ITT: Come up with ideas for new abilities and Pokemon who would get these abilities
Here's some ideas for terrain-abusing abilities

Misty Meander:
Doubles Speed in Misty Terrain
Pokemon with this ability: Aromatisse line, Rapidash (Galar) line, Gardevoir line (excluding Gallade) (replaces Synchronise), Clawitzer line, Primarina line, Araquanid line, Dewgong (replaces Hydration)

Perennial Prowl:
Doubles Speed in Grassy Terrain
Pokemon with this ability: Vileplume line, Maractus (replaces Water Absorb), Roserade line (replaces Poison Point), Lurantis, Venusaur line, Pyroar line (female), Furret line (replaces Run Away), Mightyena line (replaces Quick Feet), Dubwool line (replaces Steadfast)

Stealth Grass:
Evasion raised when in Grassy Terrain
Pokemon with this ability: Tangrowth line (replaces Leaf Guard), Decidueye line, Zebstrika line (replaces Lightning Rod), Pyroar line (male) Greedent line, Diggersby (replaces Pickup)

Feel free to post more. May do so myself later on