Interesting topic. I have to query the premise a little, though.
If there's not "one true fun", and we all find different things fun, does that mean that the game should appeal to everybody? For starters, it's a specific genre - so we've immediately turned off a section of people.
On a less antagonistic note, I'd say that in order to find out what direction to focus, you're probably looking at creating a Venn diagram of what various people find fun (within this audience - i.e. not everybody) and then aiming to create a aspect of gaming that is in the most cross-over sections.
I suspect you'll end up with vague concepts such as "progression", "stories" and "feeling powerful".