Honestly, it's a mood thing...
...well, from my POV, anyways.
Sometimes players are in the mood for Lord of the Rings, sometimes Ladyhawke, sometimes Conan the Barbarian, and sometimes Hawk the Slayer. None of these are better or worse than the other, per se, but it's more or less a matter of mood & current preferences.
From what I can guess, it's very possible to work on a certain style adventure/campaign that everyone's chomping at the bit for now (for example, let's use Eberron). Ideally, it may take you a few months to create an adventure or module, if not longer. However, during that time, the "newness" or initial hype around Eberron may have died down, and there's less of an immediate draw to stuff in that style (there'll still be some interest out there, but feasibly not as much as there was during and shortly after Eberron's debut).
As for me, my current D&D campaign is basically "find & stop the BBEG." However, much of the initial adventures for the campaign will deal with discovering that there is a BBEG in the first place. Then, it'd be a matter of finding information on the BBEG, seeing what damage he has already caused (and possibly trying to repair it or prevent it from happening elsewhere), and then it'll be a matter of actually finding & stoping him. This campaign was made in mind for using a LOT of side quests (both related & unrelated to the main storyline), and the PCs wouldn't actually deal with/confront the genuine BBEG until they were 21st+/epic level (until then, they'd be dealing with the BBEG's allies/servants).
It'd be a fair mix of adventures: running errands, performing tasks for a reward (whether that reward is wealth, information, a rare item, fame, etc.), performing good deeds, surviving an unexpected encounter, fighting a particular opponent, etc.
I'd guess that the tricky part of it is if whether or not any potential buyers feel that the module/adventure would be useful for them, or usable in their campaign. Some may want an adventure generic enough for them to adapt to their campaign world; some may want something that'll fit it snugly with their campaign world (easy to do if it's an established campaign setting like Greyhawk, FR, or Eberron; much harder if it's a homebrew); some may want one that uses new material like new creatures, items, or optional rules; some may want only core/standard rules and materials to be used at all.
It's a slippery slope, to say the least. Personally, I wouldn't want an illithid-based/illithid-heavy adventure IMC; however, another person may want a great illithid-based adventure for his/her "illithids are trying to take over the universe" campaign.
However, I would recommend that any adventure/module that you create provide multiple means to accomplish a task. If it requires an item that's acquired in the module, that's not too bad (if there's multiple means to obtain said item, then that's better). But if it requires a PC spellcaster to know & have memorized a specific spell at a specific time, or for a PC to absolutely have X ranks in a certain skill, or for a PC to possess a specific Feat (esp. one that can only be acquired after progresssing in a Feat chain), or that the PCs have at least a party member that's of a specific race, class, or AL (or any combination thereof), then there'll definitely be a problem.
IMHO, one of the most helpful things that you could do research-wise is to review threads discussing modules, settings, & adventures. There are a couple that ask about/discuss "classic" D&D modules--one of the more recent threads along that line discusses 2nd ed. AD&D "classics" (one which there are very few; and it discusses a lot of things that were wrong with those adventures).
Yes, my campaign is sort of a melting pot of everything, but it's mainly to maintain player interest in the game. Sometimes running a certain type of adventures for a few weeks or months is OK, but the players will most likely hit a point when they want a change (usually, that has involved a different game & game system); changing up the kinds of adventures used in the campaign, & the feel those adventures bring, can really help, IMHO.
Of course, you are free to sound off your opinions in this thread.
