Brian, adding Dayton to the Big East would really water down the conference. They are an average program at best, despite their lightning in a bottle weekend. The Big East, if they expand, needs consistent winners added, not the flavor of the month. With that said, if the Big East ever does decide to expand, I would bet that Dayton would get in...every league has a few bottom feeders.
I also fear St. Louis would not add much. They only became relevant under their late great coach, and this year's team is his talent.
Teams that would enhance the conference, yet stay true to their charter, are probably out of reach. Gonzaga geographically makes no sense. Notre Dame is spoken for. VCU is 'eh'. George Mason is 'eh'. Adding mid majors does not help.
Keep it add 10 teams and have even, balanced scheduling.