Where WN is right now in Terminal A - there should be some room to park aircraft / build similar “jetbridges” on the “south side” of that A3 connector. I’d think two or three (maybe four?) parking spots / gates could be built out for some more WN 737’s.
Here’s my .02 on the rest of Terminal A - UA Express (Mesa) needs to quit using A1, A2, and A7. (Not sure if they use any more “lower numbered” gates in A), but they need to be booted out and go over to B, C, D, or E where the rest of UA “world” is located. That would open up A1, A2 to be combined to make one new “normal” sized gate that could handle a 737, or 320. There isn’t much more room to allow any more competition in “A” unless this happens. HAS and the City of Houston needs to understand that not everybody living here, or flying to / from here wants to (or likes to) fly UA.
F9, AC, AS, and (not sure if B6) use the north part of A gates, but by getting rid of any remaining UA Express aircraft in “A” may help these other airlines expand. With the recent AA/B6/AS “hookup” they could have more of an opportunity to expand with new flights from “A.”
The south side of “A” is used by AA, WS, and NK (again, not sure if jetBlue uses the north or south side gates in “A.”)
The City of Houston is planning to completely renovate Terminal A at some point. Spirit wants to keep adding flights, but there simply isn't room. As for United "hogging" the Terminal A gates, that's simply not true. They pay the city just as much for those gates that Delta pays for their FOUR gates and Spirit with their three gates.
On a semi-related note, United is bringing back Gate C29 in a modified gate layout. They will utilize a gate-to-ramp system where C29 becomes C29A and C29B. That will open up two more gates for United to use. On the north side of the field, at Terminal B, Gates B85, B86, and B87 are being shifted over to create one additional gate, B87A. The gate can house up to a ERJ-175. There are also talks among station leadership to reconfigure Gate E20 to accommodate two planes, instead of one, because that gate has two jetways and can handle a wide body aircraft.
To sum it up, United is paying their share a Terminal A and is making many efforts to open up gate space around the field,