I'm being as pessimistic as possible. Typically winter gains amount to .3 from the power unit, or until the first power unit update. Assuming Honda brings another .3 over the winter, and the Red Bull chassis also improves by .3 that would mean another .6.
Mercedes says they couldn't get the most out of their spec 3 engine because of cooling issues, this may slow down progress from Mercedes relative to Red Bull if in fact Red Bull was a bit conservative with the chassis last year.
So perhaps Mercedes has less scope to gain due to the fact they're already at a more extreme state than the RB15.
Sorting out the cooling issues, plus any chassis gains, plus any engine gains, I estimate Mercedes could improve by at least .5 over last year.
If Red Bull was slower due to being a bit conservative with the design(possible due to the excellent reliability) then perhaps they have a bit more scope to improve as they push the package further and further to the brink. Combined with Honda's improvement and close relationship with Red Bull, I'm sure they will have good reliability despite pushing the packaging to new extremes. That is why I estimate that Red Bull could possibly improve more than Mercedes.
However I see it as a stretch to think they could improve by .8 which is what they would need to be on equal footing if Mercedes improves by my estimated amount.
Of course there's also the possibility I'm completely wrong, and Ferrari smashes everyone next year, or it could be that everyone more or less reaches the same performance and we have a closely fought battle for the title.
What I think is going to happen(speculation thread after all) Red Bull gets a bit closer but still slower than Mercedes, Ferrari gets closer as well. Mercedes will still have the fastest car, but the gap will be smaller than 2019, which will allow Red Bull and Ferrari to get more wins on merit. Perhaps that's all we need to have a good battle, Verstappen being .3 down still managed to get 3 wins on merit with a possible 4th with pole position in Mexico.
If he's closer maybe he can get 5 or 6 with some luck and good team work. If Ferrari is up there taking wins away from Mercedes, and Mercedes doesn't get 5 1-2's in a row maybe it's enough to pressure them into mistakes as the tension raises during the season.
In a way the Red Bull team is better equipped to deal with the stress and pressure, because they have to be inch perfect to even have a chance the last few seasons. They're prepared for the stress, will Mercedes, who until now has never really been threatened over a season be able to perform under pressure?
In 2012 Alonso took the championship to the last round despite only having 3 wins. If 2020 is as closely fought and Verstappen wins 5 he could very well win, and he'd do it in not the best car but because he was the best driver with the best team with a very good car and engine.
That would be an ideal scenario for Verstappen because he'd be winning without having the fastest car. With the fastest car you can win 6 in a row, but to get 1 without having the fastest car, very reminiscent of 2012.