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?