How do you specify the contract so the unknown unknowns can be priced into the bid and compared to each other. Given that resolving those unknowns costs about as much as the just doing the actual work.
If you want separate bidding for separate phases, bid it out that way.
If you now introduce scenarios where projects can have an unknown number of bidding processes all costs will go up to cover the work necessary for multiple companies to prepare bids.
It would just make things much much worse.