I'm very much not interested in modules that default to 4E, even if there is a conversion document. I know that sounds awful, and I apologize, but your own 3E products are competing with 4E, along with all the other 3E modules out there. And frankly, with so much good 3E stuff, there isn't a reason for me to make sacrifices.
I would be OK with modules that default to Pathfinder. I don't play Pathfinder, but it's close enough to 3.5 edition that I would just run it as-is. If there were a conversion document to 3.5, I'd take it.
Having said that, I have to admit that the best thing you could probably hope for is to hear not what a customer maybe pledges
to do but what they actually
do. And that brings me to my optimum choice. I know that in recent years I have already purchased many systemless modules (my first was Freeport). I am happy to buy something that references "orcs" and nothing more, and either leaves it to me to stat the monsters, or provides a downloadable supplement for my system.
To a small degree, systemless is even better, in the sense that I can concentrate on the plot & events, without fretting about stat blocks and trying to verify that the module author knew how to apply the rules properly.
Anyway, my kids want to watch the new Star Trek, so I'll post more later.
EDIT: OK, done with Star Trek. It's still as good as I remember it from the theater.
So the only other thing to add is -- if you do decide to provide conversion documents -- please try to make money from the module itself rather than the conversion document. I say this because once the conversions themselves start to have financial importance, it's not a far leap to protecting
that investment by locking out anyone who stats up a monster. You have good forums, and for example in the DCC forum there are many customers brainstorming how to improve modules or add interesting bits. So it would be a shame to stifle someone who produces a conversion for some obscure system, because you expected that you might produce one, someday.
Good luck. Have fun.