What you're proposing here seems to be to have ordinary fortnight challenges as we already do, plus an extra layer of topic challenge just for a single tag, which is always the same tag. That seems equivalent to just announcing that metapuzzles are a particularly encouraged tag. A better way of doing that, without needing to get consensus from the entire community, would be to announce that you, personally, will reward good metapuzzles with bounties - and if you want to reward questions instead of answers, encourage metapuzzle OPs to post wrap-ups.

  See also List of bounties with no deadlines for an example of how this sort of thing has been done here before.

What you're proposing here seems to be to have ordinary fortnight challenges as we already do, plus an extra layer of topic challenge just for a single tag, which is always the same tag. That seems equivalent to just announcing that metapuzzles are a particularly encouraged tag. 

A better way of doing that, without needing to get consensus from the entire community, would be to announce that you, personally, will reward good metapuzzles with bounties - and if you want to reward questions instead of answers, encourage metapuzzle OPs to post wrap-ups. See also List of bounties with no deadlines for an example of how this sort of thing has been done here before.

Alternatively, just create a new post here on meta with a list of all metapuzzles. The thing is, it's not really a timed topic challenge if the topic is always the same - it's just promotion of a particular tag. Which you're free to do, of course, but why mix it up with the topic challenges?