I know this won't be easy but I'm gonna give it my best shot. Grass has gained quite a boost in Gen V and I think it's possible to create a mono-type Grass team that, while probably almost definitely not able to compete with the standard metagame, will be a lot of fun to use.

I've been considering the following Pokemon:

Whimsicott
@ Leftovers / Focus Sash
Ability: Prankster
Nature: Jolly / Timid
252 Def / 252 Spe / 4 Atk
- Encore / Taunt
- Leech Seed
- Substitute
- Sunny Day

Leafeon
@ Leftovers
Ability: Leaf Guard / Chlorophyll
Nature: Careful
EVs: 48 HP / 48 Def / 200 SDef / 212 Spd (Change Spd Ev into Atk if you are using the Chlorophyll set)
- Leaf Blade
- Yawn / Toxic
- Synthesis
- Heal Bell

Lilligant
@ Life Orb
Ability: Chlorophyll
Nature: Modest
EVs: 252 SpAtk / 76 SpDef / 180 Spd
- Leaf Storm
- Hidden Power [Rock]
- Healing Wish
- Sleep Powder

Leavanny
@ Focus Sash / Life Orb
Ability: Chlorophyll
Nature: Adamant / Jolly
EVs: 104 HP / 252 Atk / 152 Spe
- Leaf Blade
- X-Scissor
- Return / Shadow Claw
- Protect

Ferrothorn
@ Leftovers / Shed Shell / Chesto Berry
Ability: Steel Thorns
Nature: Sassy / Relaxed
EVs 252 HP / 252 SpDef / 4 Def
- Curse
- Leech Seed / Rest
- Gyro Ball
- Power Whip / Level Ground

I will have Shaymin somewhere, but no idea what role it will fill yet. Support, I guess...

Shaymin
@ Leftovers
Nature: Bold
Ability: Natural Cure
EVs 252 HP / 76 Def / 180 SpDef
- Rest
- Aromatherapy
- Seed Flare
- Grass Whistle / Hidden Power [Fire] / Leech Seed

Thoughts? Ideas? Opinions?

Ooh a grass team lol. As the former Grass leader I'm excitied about this one lol. I will update soon since I am tired from work lol.

I was wondering, why do you have Healing Wish with Lilligant? It could give you a chance to set up Quiver Dances with Sleep Powder... and if you do that I suggest have either Giga Drain with Leaf Storm or Petal Dance + Own Tempo

How come Whimsicott has 4 EVs in Attack? It has no Attacking moves...

I find your choice of Pokemon strange, though as all of them share the weakness to Fire, Ice, Bug, and Flying (although Ferrothorn is neutral against the last three). With that being said, I think you should think of Grass Pokemon to counter those weaknesses instead of having random Grass Pokemon. Blissey/Chansey and many Steel types will also wall your team to no end. I feel as if I so no diversity among your Pokemon as they don't have a very wide range of coverage...

Doesn't everyone use that whimsicott set?

Now what about this guy?

Virizion
move 1: Close Combat/ Sacred Sword
move 2: Leaf Blade
move 3: Light Screen
move 4: Safeguard
item:Fight Gem
nature: Jolly
ability: Justified
evs: 4 HP / 252 Atk / 252 Spe

I definitely think you should replace Leavanny, it's not any good. It's typing is terrible and x4 weakness to flying which your team doesn't need right now. I recommend Rotom-C

Rotom-C @ Leftovers
Evs: 252 HP / 4 Def / 252 SpD
Nature: Calm
Thunderbolt
Leaf Storm
Will-O-Wisp
Pain Split

This Rotom can cover your flying weakness, or you could go with Serperior w/ Contrary and Leaf Storm perfect combo capable of a good sweep.

Tracey Leavanny can work as a surprise factor. Look at mine. It has only been taken down twice in 23 battles. I would recommend a change n set and item. As I assume that this is singles change it to this.

Leavanny @Focus Sash
- Leaf Blade
- X-Scissor
- Protect
- Flail

It is mainly a surprise factor. And if you predict right you can KO up to two pokemon with this due to Leavanny's high attack and speed.

However the choice is yours.

I would also suggest Ludicolo and Cradily in there somewhere to help with the bug/ice/flying weaknesses that your team shares.

And Fire, Rusty. Leavanny is still walled by Steel types though

Tracey, if you want we can have a battle on Pokemon Online and I'll show you what I use for my mono-Grass team

If you're going to use Will-o-Wisp you might as well use Hex (@ both Hawk and JimmyCrystal)