I think there is only a few situations that could result in us seeing sub-channels (some, definitely do not expect to see all).
1 - Directv does something for in-area local weather - many DMAs have at least one local station doing a local weather sub channel 24/7. This would make a lot of sense, especially with dropping TWC, but just throwing out the idea.
2 - Companies that own sub-station broadcasts get a transmission deal with DirecTV, not on a local basis, but national. Think ION - nothing says the owner of Antenna TV or Retro TV, etc, couldn't do a deal with the providers directly.
3 - DirecTV decides to expand on minor networks in DMAs that don't have them. We've seen a little of this, but not much, in relation to networks like CW or MY that are a sub-channel in some markets.
4 - A local station company makes it a big sticking point in a retransmission agreement negotiation - and wins. This is possible but not likely often, at least we haven't seen any examples of it to date.
Anyway you go, the big limitation is bandwidth on the spot beam satellites. Even for adding a few SD feeds, most of the spot beams are pretty full.