More expansion info!
I also see that Adventures will have tokens of some sort. Will these be a new design, or more of one of the existing kinds of tokens? Do you know how many will be included?
There are 60, 10 per player color, and they are chits.
This gives a jumping point for more speculation. It seems to me that the only good reason to have different colours for the tokens is that they have an owning player and get mixed up in some common area. Current Dominion tokens either have an owning player or get put in a common area, but not both at the same time.
My first thought is that it might be a mechanic vaguely similar to Temporum's Primitive Paradise, triggered by the presence of a certain card in the kingdom, like how Trade Route and Baker have setup rules involving tokens. Each kingdom pile could start with a token for each player, and the first time that player gains that card, they could remove the token, and maybe get some kind of bonus as a result, like say keeping the token for 1VP at game end, or powering up the card that put out the tokens (like with Pirate Ship). That would also explain why you want a mat to put them on.
The issue with that: Young Witch can make the kingdom have 11 kingdom cards. That makes putting in 11 tokens seem more friendly, since otherwise the wording+FAQ will need to be especially careful to handle the Young Witch case in an appropriate way. Although on the other hand, maybe only every kingdom pile _except_ the triggering card gets a token. So it's not impossible.
Another possibility is that players can play a one-shot to put a token of their colour on a supply pile, similar to Embargo. The token could trigger some penalty when another player tries to buy/gain the card, or better, a bonus when you do. This doesn't really explain what the mat is for though, so I'm not convinced. Also, Embargo came with 15 tokens, so this only having 10 would be strange.