Agreed on CVG, MSP, maybe CLE. WN is quite opportunistic. They don't hit a market with one or two flights, they launched CVG with a cascade of flights regionally, with additional seasonal service, and more to come this year. MSP has been steadily growing, and I think they've found success in competing against SCX. CLE for the same reason as CVG. Sure, just like CVG, it's a city that's still in an economic slump, but it's a potential hole in the network that could perform well.
I would find DTW to be unlikely likewise ONT. Maybe SNA? A lot of the WN point-to-point magic has been replaced with a distributed hub-spoke model and the tried and tested thru-flights. Why add HOU-SFO when you can go through DEN or PHX first?
There's a significant fleet problem to be solved here, too, before they start adding service to so many domestic cities.
I agree that WN adding HOU-CLE nonstop service is a possibility as WN had previously stated that it was considering adding HOU-CLE nonstop service back in October 2017 (source: https://www.cleveland.com/travel/index.ssf/2017/10/whats_next_for_cleveland_hopki.html
WN has had to drop SNA-DAL and SNA-HOU nonstop service due to WN losing slots at SNA.
I mentioned WN adding HOU-DTW nonstop service as a possibility due to (a) more passengers connecting to DTW from HOU on WN than to CVG, CLE, or MSP in Q2 2019 and (b) WN having been serving the DTW market for over 30 years whereas WN has served CVG for 2 1/2 years, CLE for almost 28 years, and MSP for almost 11 years.
I agree that WN adding HOU-SFO nonstop service is unlikely to happen with (a) WN dropping AUS-SFO and DAL-SFO nonstop service, (b) WN reducing MKE-SFO seasonal nonstop service to Saturday-only in Summer 2020, and (c) WN already serving OAK and SJC in the San Francisco Bay Area nonstop from HOU.
Here were the Q2 2019 PDEW's of HOU-CVG/CLE/DTW/MSP on WN:
HOU-DTW on WN - 45
HOU-CLE on WN - 32
HOU-MSP on WN - 32
HOU-CVG on WN - 13
While there were very few passengers connecting to HOU from CVG in Q2 2019, WN might be able to significantly stimulate O&D traffic on the CVG-HOU route if it adds CVG-HOU nonstop service since
(a) F9 was able to fill CVG-IAH nonstop flights back when F9 served IAH nonstop from CVG,
(b) F9 no longer serves IAH nonstop from CVG,
(c) UA has dropped mainline nonstop service to IAH from CVG,
(d) WN had also been able to significantly stimulate O&D demand on other routes such as HOU-CLT, HOU-SDF, HOU-MKE, and HOU-OMA when WN added nonstop service to CLT, SDF, MKE, and OMA out of HOU.
WN would also have some connecting traffic to at least AUS, CRP, ELP, HRL, MAF, and SAT in addition to O&D traffic on the HOU-CVG route if WN adds HOU-CVG nonstop service.
A post about WN possibly adding HOU-CVG nonstop service can be found at https://www.airliners.net/forum/viewtopic.php?f=3&t=1411841&p=21749577#p21749577
I had mentioned WN adding HOU-ONT nonstop service as a possibility with (a) WN having added DAL-ONT and SFO-ONT nonstop service in the last 3 years, (b) WN adding both of these routes with WN losing slots at SNA starting in 2017, (c) WN having already dropped SNA-SFO nonstop service back in November 2018, (d) WN having reduced SNA-DAL from 2 daily nonstops to 1 daily nonstop in January 2017, and (e) WN discontinuing DAL-SNA nonstop service on January 5th.
HOU-LGB, in my opinion, will likely not happen, LGB is a well-known slot land war in Asia with a rather hostile airport authority/city and acceptable but hardly exciting loadfactors. If they get a couple more, and that's a big if, I would predict PHX first. Make them go to PHX and connect or travel on a thru-flight from HOU.
While I agree that WN adding LGB-PHX nonstop service would probably happen before LGB-HOU, I mentioned WN adding LGB-HOU as a possibility if extra slots at LGB become available as there have been some individuals who recently mentioned in the Airliners.net forums that WN might pick up enough additional slots at LGB.