A good idea to have this?
or a sensible plan for Manifold?

Perhaps see also
https://manifold.markets/ChristopherRandles/what-distinguishes-sweepstakes-from

i.e. How do I answer yes and no?

Yes, it is a good idea for such markets to be available to us as manifold users.
and

No it is a bad idea for Manifold to call these real money markets 'sweepstakes' and utilise sweepstakes exemptions when these markets appear substantially different from the common understanding of what a sweepstake is.

Good idea for who needs to be clarified.