At it’s simplest: a sidewalk of chalk messages that people can walk on and is covered in notes about upcoming events in a public place.
It doesn’t have to be pretty, professional or highly visual. It could just be simple like this, but with loads and loads of messages – I imagine events would feature most.
To get more complicated there could be guidelines over what is permitted and what isn’t e.g. no commercial advertisements permitted in chalk (it should be a community/cultural initiative not a cheap way to get people to buy things) or additional features could be pursued by Council etc like chalk-commissions or 3D chalk pictures (all temporary) around special events to attract attention to the area and celebrate the Chalk Walk and the City or a particular theme. e.g. Christmas, Chinese New Year etc.
But really the more you restrict or guide people the less inviting or surprising the Chalk Walk would be. The more people that visit and leave their message on the Chalk Walk the more of an attraction it will be, so I wouldn’t recommend people are given guidelines. The most important thing is public permission for the Chalk Walk and then see where it goes…
Chalking of pavements already happens quite often, but never altogether in one space.
I think it would become a tourist attraction, an unofficial noticeboard of what’s happening in the city.
Plus you could always be sure the news on the Chalk Walk was up to date, as it would fade away in a matter of days and be re-written.