I would love to try and do a D&D timeline set of campaigns in my setting (start way in the past), where we start in D&D, jump 100 years or so and play some 1e, then 2e, 3e, ...
Y'know, some groups have kinda done that, just organically, and it can be fun. Started in one edition and migrated to the next, with or without some in-campaign event (or time passage) marking the rev roll.
My old AD&D campaign spanned 1e & 2e, but as 2e started out so much like 1e I didn't mark it in any way (though I did annoy one player by whip-sawing his exp table around, and kept an established campaign reason that elves couldn't gain 6th+level spells even after level limits were loosened up).
A GM I gamed with over the next 10 or 15 years after that ran some 2e, then a 3e campaign that transitioned to 3.5 marking the transition with a "nexus event" that got no great detail, but a lot of laughs, then re-started with 4e, jumping forward in time and having some of the old PCs feature in history. I played an Eladrin WizardMcFighter ("Like many High Elves, I am trained as both a warrior and a mage"), and I'd lampshade ed differences a lot, going on about my grandfather, who's spellbook I was working from (Crossing a river: "This is Tenser's Floating Disk, an ancient spell, my grandfather's notes speak of 'memorizing' it on occasion and casting it quickly later, but I have never managed to prepare it myself, though it is easy enough to work through the formulae and get it working within 10 minutes or so. And, once conjured, it can be controlled more readily than his notes indicated - perhaps a consequence of the more involved casting?" Spamming Scorching Burst: "Ah, yes, my grandfather's Wand of Fireballs, a precious heirloom, but it's power is not what it once was..." ) More fun than I'd had playing a magic-user in a long time.
Over the same period … actually from '93 through 2008 or so … I was also running Champions, it had a rev-roll while I was between campaigns, but I kept them in the same world, and inserted a period when superpowers nearly vanished between the two (but, as with AD&D, really not a lot of differences between the BBB & FRED - eventually I settled on Hero Lite … OK, totally off topic, now, sorry.).
I was not above pulling similar hijinks when running Encounters, commenting on how spells or items "worked differently in the distant past" (on the assumption that the Lattice of Heaven shattered in the Dawn War was actually the Great Wheel - though sometimes I wonder if the World Axis wouldn't make as much sense as the distant /past/, with Erathis' new Latic of Heaven turning out to be the Great Wheel … heck, it could be cyclical). When my players decided to skip Council of Spiders I ran a different reference-laden quest to collect artifacts to bridge to Web of the Spider Queen, it included enemies like a Lich with 'dailies' that acted a lot more like classic high-spells than usual. ;>
