There's an art here. Anyone too conservative, and Collins/Murkowski might reject. McCain is unlikely to make it back, which means the vote would be 50-49 without him, so even 1 Republican defecting would sink the vote. Depending on how Trump wants to play this, he might nominate someone who it would be tough for Dems up for reelection in red states to vote against - that would take someone more centrist.
I could also see a senator or 2 insisting on someone similar to Kennedy in order to keep relative partisan balance.