Here's one alchemical recipe for a new campaign.
1) Start with any ol' D&D-esque ruleset, though a simpler system without alot of fiddly bits probably works better here.
2) Add some supplementary rules material. You're primarily looking for new Gygaxian building blocks (classes, races, spells, monsters, magic items, etc) to drop into the game. In this recipe you want exactly two different sources for this stuff, one of which is easy to put into your game, like adding Mutant Future as a source of monsters and treasures to your Labyrinth Lord game. For the other one choose something that might be a little harder to fit into your system of choice without some work. Something like Creatures & Treasures II for Rolemaster or the monster book for Mazes & Minotaurs, the Hekatoteratos. Don't be scared to go far afield for this second supplement book.
Part of the magic of this formula will come from the tension between the two selected supplements and part will come from your own personal adaption of the second, less compatible source. Another tension, and thus more fuel to the fire, can possibly be achieved by making one of supplements an ancient text and another one a more modern invention, like using your favorite issue of Fight On! or Knockspell combined with the Best of Dragon, volume I.
With most supplements that you select, there will be some material you don't like. Like I adore the monsters, magic items, and spells in the original Arduin Grimoire, but some of the classes leave me cold. The best thing you can do would be to challenge yourself to use this stuff anyway, to stretch your own chops. You don't have to use everything in your selected books, just try to go a little outside your comfort zone.
3) Now you need some fluff to hang all this stuff on. Pick exactly three sources of campaign inspiration. Two of these sources should be recognizable as fantasy material, like selecting your favorite Conan paperback and maybe Jack Vance's Dying Earth. Note that you are picking individual works, not entire bodies of work. Pick one Conan book. Don't use the later Dying Earth books. And for Grodd's sake don't look at a fan wiki or crap like that. These books are meant to be launchpads for your own campaign, not the final word on your setting.
Your third fluff is meant to be the wild card. Pick something way out in la-la land for this one. Don't even look at fantasy novels. That'd be too pedestrian. You want something like an issue of the Micronauts comic or the movie Krull or the Principia Discordia. Or a book like Barlow's Guide to Extraterrestrials.
Using the Alchemical Method your job as Dungeon Master is to make something syncretic/synthetic that takes all this disparate stuff and coheres it together in a single campaign. Achieving this will require a lot of trial and error work as you decide what from each source to keep, what to adapt, and what to drop. The result may be a trainwreck, but it will be YOUR trainwreck.