At the moment, my main interest is with B/X D&D (in the form of Old-School Essentials). I was born too late to play it at the time (when I started, the current edition was AD&D 2nd), so I'm curious to go back to the beginnings of D&D.
It will take some time, though, until I run it, since I want to use it with Necrotic Gnome's Dolmenwood setting, which will probably arrive only later this year in book form.
Other than that, I am currently looking at a handful of OSR-titles (mainly Mörk Börg and Ultraviolet Grasslands), but I guess that would basically fall into the B/X category.
I'm designing my own retrowhatever, with the high premise of reimplementing AD&D's Player's Option series in BECMI. I'm looking at a lot of 3.5 and Pathfinder 3pp for inspiration-- Tipsy Tabby Publishing's Overhauling Multiclassing and Rogue Genius Games' Genius Guide to the Talented $CLASS series especially.
So I'll run Rules Cyclopedia or AD&D 2e Player's Option pretty happily... though 3.PF is still too much work... and I'll play any of the three under a flexible and generous DM.
I have a friend who only wants to run D&D 3.5 "Core Only"... she invites me to play, I express my enthusiasm, and then she says "Core Only". She doesn't understand why I immediately back out, and I don't understand why she doesn't say "Core Only" up front, and neither of us will ever learn.
Mine is going back to 3.5 reduce HP, redo monster math and add some 5e improvements -NeoVancian and advantage/bonus dice for buffs-. Also streamline things here and there.My pet project is advanced B/X with microfeats and 5Es skill system and bounded accuracy.