I honestly can't see Adele singing the Theme, these nothing to add to what she did with Skyfall regarding sales and Awards. It's the biggest selling Bond song of all time and the most awarded.
Unless it's an extension of the previous song.
Though her album now seems to be coming out in the 2nd half of the year and Bond is out towards the end of the year. So could be.
Other names mentioned on Bond forums are;
Sam Smith
Ed Sheeran
U2
Swift (sure hope not)
Paloma Faith (would love it)
Elton John (would love it)
Susan Boyle
Andrea Bocelli
Sam Mendes recently said he knows who will be singing the theme song. Can see Eon wanting another hit and to do that they need someone who can sell in today’s market. Billboard says streaming is now more important than radio for gaining sales. So, take Elton, Boyle and Bocelli of the list, these don't have big streaming or radio and a good chance of a flop.