Activity sure exploded in the few weeks I was gone! I'll try to answer a few topics. Let's see...
Grass-type ability
I'm not really sure. I'll probably go with Bulletproof if that counts. A lot of this might stem from something silly that happened: I was in the Battle Maison with pokémon from my in-game team. My chesnaught was walled by a Bulletproof quilladin, ironically! I guess my moveset must've been Spiky Shield, Leech Seed, Seed Bomb, and Bulk Up or something. Haha! It probably helps that I absolutely adore the chespin family.
Grass-type mega
I've never used a mega. Well, that's not entirely true, since I used a mega alakazam "competitively" just because it was shiny. But yeah, my in-game runs never allowed me to use a mega (Kalosian pokémon only in X and Y, unevolved pokémon in OR). That said, I suppose I like Mega Sceptile the most for its design.
New grass-type move
Hmm, good question. Maybe something like a reverse entry hazard?
Name: Heal Sprout
Category: Status
PP: 10
Power: --
Accuracy: 100
Target: User's side of the field
Description: The user grows a small field of sprouts that heals some HP upon switching out.
Or maybe it should be switching in? I don't know, I haven't figured out the details. :P
Let's not let this garden wilt due to neglect! How about another sub-topic?
Create your own Grass type attack! I'm fairly certain this hasn't been done before, and even if it has, well...embrace your creativity!
Name: Calming Seed
Category: Status
PP: 10
Power: --
Accuracy: 100%
Target: Anyone adjacent to user.
I was looking over existing Grass type moves and realized that, for all the Status devastation they can cause, there's only two that can put opponents to sleep, and both have undesirable accuracy, with Grass Whistle at 55% and Sleep Powder at 75%. So I wanted to make a Sleep-inducing move that was more reliable. The result? A sort of parallel to Worry Seed.
Well there's Spore, but that doesn't exactly have widespread distribution huh?