The basic idea is that each player can bring in a character from whatever edition he prefers.
I think this might be extra difficult if playing face to face, but for Play by Post games, doing the maths for quick conversions should not be that hard.
Of course, the more editions you allow in the game, the more complicated things become.
I don't think we need to worry about every variable. In some cases players from a specific edition might be at an advantage, but if we get the biggest issues out of the way, then the rest should be things you can brush over.
I think the most important thing is that the DM decides on a baseline system and then converts effects on the fly.
Things to work out:
- Ascending vs. Descending AC: This is pretty easy to work out.
- Saving Throws: These work very differently and the categories have changed many times over the years.
- XP: I'm thinking about using Milestones or half milestones here, but this doesn't work so well for the TSR era editions. I might come up with a system to convert milestones to XPs for those editions. You don't want your AD&D Thief to gain levels at the same rate as an AD&D Wizard.
- Ability Checks, Skill checks, Proficiencies: This is also something you need think through.
- Difference in numbers: 5E has the lowest number ranges, while 3E or BECMI can get crazy high. This is something to take into account and maybe have some conversions for.