Hiya!
I have been toying with this idea for a while. I am thinking of writing a setting neutral campaign able to support several different systems. ie: combat encounter x takes place in a volcano. Use these monsters:
5e fire elemental of x size
4e: fire elemental of x size
3.5 fire elemental of x size
Pathfinder fire elemental of x size
Gurps: Fire elemental basic
Etc,
How hard would that be to get published?
If you publish it yourself, go for it. Otherwise...well..."difficult" would be my guess. At least the "publishing" part.
Writing it is significantly easier. I've had adventure ideas that I thought would be cool for multiple game systems. Usually, actually. It's more rare for me to conceive of an adventure that is "specifically" for System ABC or whatever. Anyway, I have often just written down the "Common Language System" info.
Oh, right, I created a multi-page "RPG system" that I call the Common Language System that has a few basic formulas for gauging power levels and whatnot, and I use a Common Language to describe these 'power levels'. I did this a couple decades ago when I was doing a lot of "game system conversion stuff".
Now, when I write an adventure I think might work for, say, Powers & Perils, AD&D 1e, and Astonishing Swordsmen & Sorcerers of Hyperboria, I can just write down "Monster: Mid-Power Goblinoids" or "Trap: Easy Difficulty w/ Dangerous Outcome [Incapacitating]". Then, if I happen to run it with P&P, I know "Ok, Mid-Power Goblinoids would be about.... 6 Goblins". If I'm running it with 1e AD&D, "Ok, Mid-Power Goblinoids would be about... 3 Hobgoblins". If it's the trap, for P&P "Trap: Paralyzing Gas (Difficult); BL 11; Paralyzed for 1 minute per point failed by)".
When I'm doing a "conversion" on the fly, I almost always just use the "word" and use the systems interpretation of that word. So if I see "8 Kobolds" in a 1e adventure and we're playing 5e, I just sub in "8 kobolds" from the 5e MM. If I see "Poison; Save at -4 or take 2d10 damage" in 1e, I just use "Poison; Con Save, DC 14 or take 2d10 damage" (I just use "10" as the base number for any DC and then adjust as it feels right).
That said, when writing and PRINTING an adventure I write, I make sure to have 'post-it sized blank spaces' under each "entry" (encounter, room number, etc). An adventure called "Tomb of the Iron God" (
Review of Tomb of the Iron God - RPGnet RPG Game Index ) actually did this!

There's a nice blank space where the GM can write his own notes; I just used Post-It stickies with my conversion info on it and slapped it down. I ran that adventure using the Dominion Rules system (
Dominion Rules - Home ) and it worked very well!
^_^
Paul L. Ming