A 1-1 draw at Elland Road in the second leg saw Millwall defeat the Whites 2-1 on aggregate but Bates is refusing to be downbeat and is aiming for a title tilt next season.
He told the club's official website: "(Manager) Simon (Grayson) came in to see me afterwards and we went through every player on the books and what we want to do.
"I had a staff meeting first thing on Friday morning and I told them 'last night has gone. We're now planning for promotion next season and that planning starts now. This is the first day of our title-winning season.'
"We had a traumatic season because for the second one in succession we had a change of manager which is always unsettling.
But next season we start properly. We have a good manager, a good playing staff which will be strengthened and great fans.
"We're very lucky here. We have a passionate fan base, we have big plans for the club, and we've had a set back. We are a year behind schedule but we'll put that right next season."