Most, like Jeremy Hunt (championed Mark Garnier MP with @Emmabarnett below) have refused to say what they'll put on the table in such a negotiation.
Let's consider the options.
"If you re-open the Withdrawal Agreement and remove or significantly limit the backstop, that will create a deal the UK Parliament can pass."
This a simple trade, backstop out to avert No-Deal.
(1) It values a guarantee for the Irish Border; &
(2) It values the credibility it would lose by reversing a position oft-repeated at national leader and Commissioner/EU President level.
(1) Theresa May either didn't ask for this, or didn't ask for it properly;
(2) The EU's resolve on the issue has weakened as No-Deal looms closer; or
(3) The EU were bluffing the entire time.
(1) Trading the EU something it wants; or
(2) A heretofore undiscovered creative solution.
He advocates the UK offering costs of any border infrastructure/updates Ireland may require in exchange for removing the backstop.
This assumes Ireland is significantly motivated by the construction cost of some border posts.
Setting the legalities and ethics of this aside, this is another commercial assumption.
This isn't to say no such trade off exists, only that it hasn't (to my knowledge) been identified by candidates or commentators.
Charting an undiscovered safe route through red lines previously considered impassable is the best feeling in the world.
Unfortunately, it's also really hard.
The UK does not want to sign up to a backstop it cannot legally unilaterally exit.
Them's the breaks.
As always, I welcome contributions from others more experienced or simply more creative than myself. /end