My only must-do is I must enjoy myself. Around 50 days in the park the 3 years I was a pass holder. Few days did I get in all my favorites at any of the parks (minus DHS, where it's easier to hit ToT, RnRC & ST). I can't think of a day I didn't enjoy immensely. With the new rides, it will be even harder to do everything every park day, but that's a good thing!
10 years ago we were entering the worst summer for the US economy in over 50 years. Go back 20 years. Lots has happened around the world, in the US, in FL more specifically (epicenter of housing bubble?), and at WDW. So much has happened, it's hard to determine what is the real cause for different changes. Except when they changed pass holder pricing from 2015 to 2016. Pretty sure that had a big impact on the slight attendance drop that year.
Epcot's attendance is more than it was in 2010. As is HS. Both are not up as much as MK or AK, which are both up close to 25% over that period. I'd more say Epcot and HS have not had the explosive growth MK & AK have had. DHS will get that with TSL & SWGE. Epcot, not so sure. However, all the added attendance over the next 4+ years will naturally lift Epcot's attendance. Many of those going to WDW for SWGE will make a day at Epcot.
I liked the holiday magnet, and made a special trip for it. I see the Flower & Garden ones on a lot more cars than I'd expect. Passholders complained for perks, got magnets.