I would love a Denney/Frazier comeback!
They've been coming back since her devastating knee injury and they made strong strides initially in 2016-2017, but perhaps had overdone it a bit on the comeback trail and ended up imploding in the sp at 2017 Worlds. They had to make another comeback from that setback and they did because they hung tough and believed in themselves. They apparently received crucial support and encouragement from their families, coaches and USFS, and more importantly from each other. They've made stronger and stronger showings, since last season. Reminder: D/F were second at U.S. Nationals earlier this year, and they had wonderful programs choreographed by Charlie White. They again have good programs this season and they took advantage of opportunities on the GP. I would suggest that they are steadily building their confidence. Unfortunately, respect and support from fans seems to be the bigger ask. Meanwhile, D/F have my support and best wishes, as always! ♥
If you mean that you hope D/F come back to win U.S. Nationals, that's quite possible. Hopefully, the battle will be joined by Ash & Timothy as well, with good outings from all U.S. teams. As I've mentioned a few times in this thread, I love pairs, and I especially enjoy watching U.S. Nationals pairs events many times over!

It's not always about being heavily invested in outcomes. It's about enjoying the moments and the memories and having compassion for the athletes and all they go through to pursue their dreams.
Combine the best skills of the top 4 US teams and we MIGHT have one team that could challenge for an international podium. Unfortunately, tag team skating isn’t a recognized discipline.
You could say that about any skaters in any figure skating discipline in any country though.
The risk is that judges then see them as a lower level team, and keep the PCS below the top teams.
Reminder: Peng/Jin purposely have been competing double salchows, which didn't prevent them from winning Skate America, sadly, nor from making the GPF, to their benefit. Calalang/Johnson implemented a similar 2-salchows strategy at Warsaw Cup in the fp (in addition to re-positioning jump placement in their sp). So, I think strategies will depend upon the particular team and their individual circumstances.
C/J and D/F are my two favorite US teams right now.
D/F have always been one of my favorite teams and C/J had me at hello, as did Ash/Timothy! But did I say (I think I did) that I love pairs and I enjoy watching all U.S. pairs teams, and I support all U.S. pairs teams! Plus, I enjoy, admire and have favorites among a number of international pairs teams.

The thing is to try and support the teams you love come rain or come shine! And try not to be a fair weather fan. No career in skating is without its ups-and-downs.
So far the USFS approach to improving our pairs has not paid off yet... USFS needs to continue to devote serious effort to getting our pairs to be able to compete cleanly. Maybe sending to train in Canada?
You must be joking. It's not up to U.S. fed to get teams to compete cleanly. U.S. fed needs to be supportive and offer guidance and financial support, and help in other ways as they can and have been doing. But competing cleanly is more up to coaches and the skaters themselves to figure out. Re 'sending U.S. pairs teams to train in Canada,' IMO that's not necessary. Just because it has worked for some U.S. ice dance teams is neither here nor there. Sadly, training in Canada did not work out particularly well for Castelli/Tran for a variety of reasons, perhaps some reasons having little to do with their coaches.
Where skaters train is variable in this current global skating community environment. Lots of international skaters train in the U.S. It's more about the coaching relationship and training environment working well on an individual basis, no matter where skaters train. Detroit/ Canton was once the place to be for ice dance teams, and now it's Montreal. But there are still some good training locales for ice dance in Novi, MI and in Wheaton, MD, etc. Florida has some great pairs training facilities, as does Colorado and California. Berton/Brubaker are currently coaching pairs in his hometown of Chicago.
I don't know what the U.S. approach is exactly or is supposed to be to improving U.S. pairs teams. However, U.S. fed has certainly been paying more attention to improving their pairs discipline, and it definitely has paid off, so I'm not sure why you think it hasn't. Rome wasn't built in a day.