A focus city operation for DL in IND could definitely happen. Focus cities are built on local traffic, not connecting traffic, so it isn't a death sentence if there are 2 focus cities within a few hours of each other.
Having connecting hubs within a few hours is not a recipe for success. Just ask US when they had hubs in BWI, PIT, and PHL at the same time.
The situation with DL and IND is different than US Air's.
I was closely involved with the NW operation in the Ohio Valley, going back before the NW RC merger.
We believed that IND had an ideal location within the the NW hub structure. DTW, MSP, and MEM allowed for omni-directional one-stop connections to nearly every on-line city.
This gave NW an advantage on elapsed-times for numerous city pairs in every direction from IND. This was a significant advantage in the days of CRS when elapsed time was one of the primary metrics that drove what air line went to the top of the Travel Agent CRS CRT when 95% of airline tickets were booked over Agency CRS.
After RC started with four DTW flights, additional flights were added to MSP and MEM. This provided synergy for NW to grow marketshare, despite a poorly initiated RC/NW merger.
By the late 80's NW had grown to be the 2nd largest carrier from IND, only surpassed by US, with their small IND hub.
This continued through the 90's and 2000's, as NW added additional domestic connections, modernized facilities, the KL/NW JV, improved customer service, and numerous corporate agreements. NW maintained an enplaned passenger share larger than it's share of available seats, with higher RASM than competitors during the period.
This was the situation that led NW to build a focus city in IND in the Mid-00's.
That went by the wayside as DL wisely (in my opinion) elected to focus on their hubs, and merging the operation. The removal of RJ-200's, and the DC9-30 from the DL fleet also left DL with fewer options for the city pairs added by NW's focus city initiative.
Today, DL is still reaping the benefits of this of this 30 year history. Couple this with DL's historic strength over ATL, leads to a situation where I believe DL can add additional local origin flights from Indy to a few more spokes, leading the re-creation of what NW had in their focus city.