It appears that a bunch of DEN flights that have previously been summer seasonal, or were initially announced as summer-seasonal, have been extended to year round, including:
In addition to this, DEN-OKC will be running daily through the winter (random thing I noticed). Also, TPA and RSW will be running daily, and MCO will be running 2x daily. One noticeable missing flight from DEN's winter schedule, however, is DEN-SJD. DEN-PVR is returning though.
LAS appears to have remained quite stable during this extension, with only MIA not returning, both TPA and BNA appear to be extended to year round (TPA assumingly coming at the expense of LAS-MIA), SFO returning for the season, and new additions YYC, SMF, GSP, ORF, JAX, and GEG all staying through the winter. (Contrary to what the LAS wikipedia page says, IAD is staying as well)
At MCO, it looks like ABQ, TUL, AUS, and SAT have been extended to year round. CLT, BUF, and BNA are all running daily this winter. SAN, TYS, MSN, and MSP are back for the winter, but it looks like CID, SFO, and PHX are not returning. It also looks like DSM also will not be running through the winter, but it may return next summer.
I randomly noticed ATL-SFO is returning, but SJC-ATL is being cut.
TPA and MIA were both huge surprises to me in the sense of how many routes aren't returning this winter.
TPA lost GSP (was this route cut before it even began? It won't show up on their website even for the fall), CMH, IND, MCI, and STL, managing to only keep MKE and MSP from last winter. However, DEN is going back to daily and LAS is staying through the winter after having previously been a summer-only flight, and will be daily.
The biggest surprise of this extension, IMO, was the huge cutdown at MIA compared to last winter. LAS, BUF, CVG, CLE, DTW, MKE, PVD, and TTN are all not returning, leaving LGA as the only flight to return from last winter.
RSW fared better than TPA and MIA in regards to cuts, with CMH, MCI, OMA, PVD, and STL not returning from last winter. However, CVG, CLE, DEN, MKE, and PHL will all be running daily this winter, and RDU is beginning in November.
PBI-PHL does not appear to be returning, but TTN-PBI and ISP-PBI both are running daily through the winter, and RDU is beginning in November.
JAX did better than I would have expected in terms of cuts. BUF, CVG, MKE, and SAT won't be running this winter, but DEN, LAS, AUS, ORD, MCI, MSP, PHL, STL, and TTN will all be running through the winter.
AUS did better than I would have expected as well. CVG and PHX were cut, but ATL, CMH, JAX, SNA, MCO, and PHL were extended to year round.
SAT did not fare as well, however. ATL, COS, PHX, SJC, and IAD all appear to be cut, and only CLT, SNA, and MCO were extended to year round.
RDU did exceedingly well, with no year-round flights being cut and DEN, IAH, MCI, MSY, SAT, SAN, SJU, and TTN all being extended to year-round. Also, the schedules were loaded for MBJ, PUJ, CUN, RSW, PBI, and PHX. MBJ, PUJ, and RSW each running 2x weekly, and PHX, CUN, and PBI each running 3x weekly.
A second PHL-SJU and MCO-SJU frequency will both be running on Tu/Th/Su through the winter. In addition, ATL-SJU and RDU-SJU will be running through the winter.