Floriana Lima left Supergirl as Maggie Sawyer, but why couldn’t she reprise the role to introduce Kate Kane in the crossover?
At this point, it’s worth noting that the Maggie Sawyer who could be romantically attached to Kate Kane doesn’t have to be the same Maggie who nearly married Alex on Supergirl.
In fact, she could be the Maggie of Earth-1 who introduces Oliver and Co. to the Earth-1 Kate Kane, allowing the Batwoman show (if ordered to series) to potentially cross over with Arrow, The Flash, and Legends of Tomorrow without hopping universes like Supergirl.
Kate is described as somebody committed to social justice in Gotham City, so she and Maggie could have a lot in common as law enforcers, just as Maggie and Alex did.