The guy who originally leaked this change saying that the IP is only relevant this year makes me think that IT is out. Pennywise is almost as timeless a horror icon as Jason or Freddy. You could do a house based on IT next year, or in 5 years and nobody would bat an eye.
I think the new IP is more likely to be Us. I dont see that having much longevity in the public consciousness beyond a few twins dressing up for Halloween this year.
Edit: You know what, after thinking about it, him saying the new IP is 'only relevant this year' could be in reference to WB not doing their own haunt thing this year, allowing Universal to use IT this year only.
Either option sounds good to me honestly, and I am enjoying a bit of the 'unknown' once again in our speculation for 29!