I don't see why they would move so few teams from open up.
Basically what you suggested would be keeping the same teams moving out of open as past seasons, and simply splitting what would have been IM in half.
If the purpose is to have skill more evenly divided between divisions, that would really do nothing to ease the gap between high and low open. What I suggested in my edit seems more logical to me. That being:
9-10 team invite made of surviving invite teams, rebuilt invite teams, and two moveups from s14 IM.
~12 team Main formed by ranks 3-12 from s14 IM and 1-2 from s14 open.
~28 team IM from the bottom few teams of s14 IM and the top ~25 from s14 open, except ranks 1 and 2.
50+ team open from remaining open teams and new teams.
This way the expected "top open" would be moved to IM, hence lowering the skill ceiling of open and allowing low teams to stand a better chance.
Of course it would also be best if some judgement could be made about placing new teams with previous invite/IM experience in IM to further remove "top open", but i doubt that will happen.