Right now, here's the Mid-State
Decatur Central: 1812 - solid 5A
Franklin: 1571 (and growing fast) - 5A
Greenwood: 1193 (and holding at that number - Greenwood is largely built-out) - 4A for good
Martinsville: 1286 (and not really growing) - 4A for the long haul
Mooresville: 1391 (room for some growth, but it doesn't seem to be as dramatic) - 4A for a while
Perry Meridian: 2385 - 6A
Plainfield: 1823 (and growing fast) - 5A but likely 6A within a few cycles
Whiteland: 2021 (and growing fast) - 5A but will soon be 6A
Whiteland is on the cusp of 6A, and there is a LOT of unbuilt land in Clark Township right off I-65 that will explode soon. I wouldn't be shocked to see Whiteland be 3000+ in a decade.
Meanwhile, you also have Conference Indiana
Bloomington North: 1664 - 5A
Bloomington South: 1733 - 5A
Columbus North - 2219 - 6A
Southport - 2391 - 6A
TH North - 1575 - 5A
TH South - 1651 - 5A
And some other schools that could be in the mix
Center Grove - 2754 - 5A (independent)
Franklin Central - 3223 - 6A (and growing very fast)
Long-term, I think what we could see is some shuffling where the larger schools in the Mid-State and Conference Indiana join forces (and possibly add CG/FC, although I wouldn't be surprised if the MIC is a long-term destination for FC just because of geography).
We could see something like this
Center Grove
Whiteland
Columbus North
Decatur Central
Franklin
Plainfield
Southport
Perry Meridian
And
Martinsville
Mooresville
TH North
TH South
Bloomington North
Bloomington South
In this scenario, Franklin, Plainfield & Greenwood are the wild cards - I can see Franklin growing to the point where it fits better in the 5A/6A league. Plainfield fits better in with Martinsville/Mooresville geographically but will likely top 2K students soon.
Greenwood is basically left homeless by this shuffle. I expect to see the Woodmen make a run for the Hoosier Heritage Conference, especially if Shelbyville or Delta decides to go to different leagues and they need an eighth team. They don't really fit geographically, but they do fit enrollment-wise (at least for now).