I think the director of the anime mentionned that they pick the ending theme based on what Araki was listening to at the time, meaning something from 1999 sounds unlikely.
This is what I've heard too.
NO Idea what they might include then. However, if I got to pick an ending theme for the series, my choices would be:
1st Half: We Built this City by Jefferson Starship
2nd Half: Killer Queen by Queen
I also have another wish for one:
Dancing in the Street by Mick Jagger and David Bowie.
It would ONLY work tho if they recreated (A shortened version of) the music video with Josuke and Okuyasu in place of Bowie and Jagger.