I think she might become an independent but still caucus with reps (like Sanders does with the Dems), so not much change.
All anti-Trump members of Congress should just switch to being Democratic, or at least Democratic-caucusing Independents.
There is no future for them in the Republican Party. As Democrats they can form a new Centrist Caucus with some of the existing Democrats and hold a bit of leverage when they make the different in who is in control.
Would she keep her seniority if she defects to democrats?