GAZ 1 Grand Duchy of Karameikos and much of the rest of the GAZ series was great, if a little cluttered and nonsensical if you tried to tie it all together. I rather liked the related Top Ballista! setting for Mystara as well, with the skygnomes and their magical biplanes. And player character sphinxes. All in a flying city that could show up anywhere. It was kind of awesome, but very mechanically tied to BECMI for all the races introduced (and used races as classes, of course, like the rest of the system). I did try doing 5e with a GAZ 1 campaign using Keep on the Borderlands (the 5e conversion) but then we got sidetracked with the Waterdeep 5e campaign with a more consistent DM.
Amazed how much mileage I got out of Tomb of Annihilation and the related Tortle Package. I ran a total of four campaigns using it to varying extents for different groups over the past couple years, including one play through of the main module and one full exploration of the tortle island. For the sequel for the tortle group I'd like to use Qelong from Lamentations of the Flame Princess as a follow up, situating this weird fantasy Cambodia in the Samarach area of the Realms, but off and running with something else right now with them. I found the Innistrad MtG crossover kind of appealing as a setting, but I took it in kind of a weird direction inspired by Franz Kafka's Metamorphosis that was maybe less appealing to my players who are really into playing D&D when we are there to play D&D and it definitely needed fleshing out if you weren't going to do the suggested meshing of Barovia with it.
Have also found Testament from Green Ronin for 5e very inspirational for doing Bronze or Iron Age D&D. And I really like HR5 A Mighty Fortress from the old historical reference series from TSR for the 30 Years War and Reformation periods.