I'm a lead farmer, mother lover!
I doubt it, for the exact reason you stated - They don't own the Halo IP anymore. Selling the rights to Microsoft (as part of their contract to leave Microsoft and be their own developer), completely outs them from allowing to use anything related from the Halo Universe. That is, unless they have platform exclusive content, in which case they may be able to bid Microsoft to place a Halo concept into it (Such as a Spartan character model or something akin for whatever they decide to do).