I've been thinking on D&D Classics recently, the various PDFs of old D&D materials from past editions. As I've reflected before, back in the day I made some mistakes in my purchasing - I should have skipped a great many of the rulebooks and splatbooks, and not dabbled in so many settings, and instead gone deep into a few chosen settings and bought more adventure modules. D&D Classics obviously offers a way to redress some of that.
Earlier this year, once the OGL fiasco was cleared, I bought up the remaining Spelljammer PDFs to complete my collection of that setting. I've now started gradually collecting the Dark Sun PDFs, and then I intend to invest in a set of the Ravenloft 2nd Edition books (unlike the other settings, Ravenloft had a really big presence in 3e, but I'm not going to bother with that).
I also intend to purchase a bundle of the Historical supplements, as those are regarded as being surprisingly good, and then a smattering of 1st Edition (and a few 2nd Edition) adventures - by no means a complete set, but those items that are regarded as being especially good.
But all of that is a long (ish) term project - I've had no time to read any of the Spelljammer materials, so I'm in no hurry to collect the rest. And, of course, if I'm not actually playing...