Just come back with a modern song and play it like you were never even gone. Not with some nostalgia bullshit fan message of a song like SES or SNSD or even worse, with a cover album, like Brown Eyed Girls. Nobody wants that shit.
Fans want a song that'd remind them of the days their faves were at their prime, they want to listen and say "See, this is why I loved them", not a sad song about "long gone days".
If you ain't gonna do it like that, just don't bother coming back