I thought this was a very interesting idea, and wouldn't mind actually seeing it come through myself. A player posted this in my forums and I liked the idea so much I told him I would take it to you :]

1. Speed Upgrader:
   While on this block, your speed is increased to of a swiftness potion!
   Only works for owner and his/her clan
   Recommended Myth-mart price: 2 for 10g
   Is a Brown Giant Mushroom block.
2. Speed Downgrader:
   While on this block, your speed is reduced to that of soul sand on ice.
   Works on Non-owner and non-clan players.
   Recommended Myth-mart price: 2 for 10g
   Is a Red Giant Mushroom block.

Hope you like the idea for the flags!
Thanks for reading,
-moneybagman

Added potion flag, lets you give players inside a field a potion effect