Grassy Terrain:
With the addition of Grassy Surge Thwackey and Grassy Slide, there is a ton of potential to make Grassy Terrain teams in the same vein as other gimmicks like Sun and TR. However, Terrain isn’t that huge of a buff in most areas, and does not currently have any true abusers in the same way as Swift Swim, Weather Ball, Solar Power, and Solar Beam tend to be overpowered when in their respective weathers. Regardless, I’ll open the discussion of what I’ve seen and experimented with so far:
The Setter: While only an NFE, Thwackey still brings pivoting, priority, and a few other niches to the team. The biggest downsides are its mono-Grass typing and awful bulk if it’s running Terrain Extender, which it more or less should depending on how dependent the team is on Grassy Terrain. Knock Off, Taunt, Wood Hammer, and SD are also interesting picks for it. I don’t think Thwackey is as much of a liability as Vulpix was for Sun teams, but at the same time Sun had much better abusers to warrant it. 
Grassy Glide: This attack is +1 priority at 92 BP after the terrain buff, making it one of the strongest priority attacks in the game. Thwackey itself should probably use it given its poor Speed and vulnerabilities, and so should some of the team’s main wallbreakers. CB Hustle Flapple is likely the best, as it reaches an absurd power that is no longer set back by its middling Speed. This thing is so strong that it 3HKOs HDB Scyther in the terrian; that means a x4 resistant switch-in can only come into Grassy Glide once, as if it tries again it will be KO’d! This makes Flapple an awesome revenge killer and late-game sweeper, with the only obvious downside being the low accuracy of Hustle. Past that, its Dragon/Grass coverage with U-turn to boot makes it a very rewarding mon if played well / when it doesn’t miss. Other potential abusers include the Gourgeist forms, Treveant, Abomasnow, and Leafeon to name a few that could be worth exploring. 
Support: Having likely at least two Grass-types on the team can be an issue defensively when the typing has a ton of weaknesses to cover. Thankfully, Grassy Terrain’s benefits of passive recovery and nerfing Earthquake pair nicely with additional support for the team. Lanturn is a big one as it can stay in on opposing Sandaconda and Piloswine and Scald, as EQ is no longer an issue. Its ToxicTect sets also appreciate the 12% recovery from the grass and Leftovers, giving it a ton of longevity. Silvally-Fire is a more offensive pivot that can help target defensive Grass-types while also having a boosted Grass Pledge against its usual switch-ins of Wishiwashi, Lanturn, and Sandaconda. Piloswine can run High Horsepower and still retain its Ground coverage while being a good rocker for the team. Garbodor likes the recovery and resistance to EQ not for Grounds but for physical attackers it can wall a lot better now like Toxicroak, Sawk, and Stunfisk-Galar (which is a Grass-type under terrain, and thus is weak to Gunk Shot lol). Here’s a team I’ve used exploiting Grassy Terrain. These are the main pros of Grassy Terrain teams, and I can see them getting even better with potential drops and bans. Speaking of potential bans…
