Compare/Contrast Waco/Temple/Killeen with the Los Angeles Basin/SoCal.
In CenTex, we've got ACT serviced by AA Only. And Waco has struggled to have more than one carrier for 30 years (that I know of). I did fly on Rio once from Waco to Houston circa 1983 maybe. I don't remember the circumstance but it was the first solo flight that my little brother and I took sans parents. Funny now but we must have been like 13 and 11 then.
Then we've got GRK serviced by AA/DL/UA. Three different airlines see fit to fly into Killeen. Of course one of them - Delta is probably heavily influenced by US Military contracts/personnel movements.
In Los Angeles, we have 5 major airports and a population of 12-15 Million depending on how you cut it.
LAX - fly anywhere in the world, major world hub
SNA - domestic only in Orange County
BUR - domestic only in Burbank
LGB - domestic only in Long Beach
ONT - domestic commercial flights but International Cargo flights
Then, the outliers would be OXR (Oxnard), PSP (Palm Springs) and SAN (San Diego) which are all domestic as far as I know. I don't think there are any international flights in/out of San Diego other than cargo.
Anyway, quite a few airports in a small space but a massive population. And there are all kind of pricing games you can play which is why one time when I needed to travel to Waco, I flew from PSP-IAH-GRK and back because it was several hundred dollars cheaper than flying from LAX, BUR or SNA even though I lived 5 miles from the Burbank Airport. I drove 115 miles into the desert to save $300+. And again, just like this time, I flew into GRK because it was $300 cheaper than ACT.
I think expanding GRK is probably the right move because it's already setup to handle larger planes. I did help load a C5 there once upon a time.

Of course if it got too big then you'd probably have conflict with the military traffic there.
As it stands right now, with it costing an extra $300 to fly to Waco versus Austin or Dallas, and the extra time flying + waiting for a connection to another flight, there's no good incentive to fly out of Waco.
If it costs $300 and 2 hours of my time to get the connection from DFW-ACT, usually, I'll just say to heck with it and rent a car at DFW and drive to Waco.
The only airline situation that is a little different in CenTex is Delta because they service GRK from Atlanta. So, if I was headed east, or to Europe or something, that would probably be appealing. I just wish Delta would add flights to Salt Lake so I could go west from Killeen without having to go through ATL.
Enough of my CenTex airport rant. Frankly, I don't see any reason why they couldn't build a massive airport north of Belton and have it be another DFW type hub for airlines who are looking to expand into the southwestern US. It would be a perfect hub for somebody like JetBlue. It could be like Phoenix. I think lots of people connect through PHX but I don't think that's anybody's final destination.