TAG! Your it!

[vengeful]

Tag. Very simple and very fun. Everyone runs and one person is it. If tagged it could be normal and your now it. Or if tagged you could be out and when it gets down to the last person they get a prize! It could be in a pokemon park or in a town.  In a cave in the dark, or just somewhere you can run.

Weekly or weekend event. Sign-up only with prizes or not.

[LeoReborn]

It's a very simple and basic idea, but full of fun.

Not to mention it wouldn't be hard to code in.

I like this idea, its a good way to escape the main flow of the game. (leveling, badges, etc.)

Like you said, it should be sign-up only, a prize would be nice.
This means that there would have to be 'rooms'. In this case, Id suggest parts of the Miryus map, closed of to create a field.

[Monzta]

How do you "run" away when everyone walks at the same speed?

[vengeful]

With running shoes! i mean yes it is the same pace but you dont know if they will try and trick you or just go after someone else.

[Bing]

How do you "run" away when everyone walks at the same speed?

Make a tiredness meter.  That refills when you walk and drains when you run.  And when it empties completely have 3 second delay until it starts to fill again.  This would solve the walking at the same speed problem. 

Also you can include barriers.  Cover like trees where because of the top down view you can't be seen that well.

[Hiccup_Hero]

or the tagger has an adv of slghtly faster running during the game (you could do hide and seek tag too)