Oooooh Phantom Menace. Back when we were all so innocent.

I contributed to the LOTR marathon - I'd do the same for a Pirates of the Caribbean marathon or Potter marathon or Bond marathon. Or historical epics like Gladiator/Troy/Alexander/Kingdom of Heaven.
Or a marathon could be thematic e.g. all three King Kongs for a great compare & contrast.

Or alternatively, DiF could nominate what they'd do for $x. Crowdsourcing. Transformers 3 or Underworld movies might attract a higher price because they're more painful than something fun like Avengers or a Nolan film.  And it's up to us forum members to meet that target.

So there are two alternatives: push or pull. We the forum member push certain movies and DiF choose or DiF nominate and forum members choose.

Another idea... DiF could threaten to have an Adam Sandler marathon and forum members could pay NOT to have it.

One option that's been working really well for the filmjunk podcast is they still release their normal weekly shows, but every couple of weeks they'll release a "premium" podcast, which they will charge a minimum of 99 cents for via BandCamp. Examples in their case have been things like doing the Back to the Future trilogy, Batman movies, Star Wars, etc.
Kind of trickier in your case cause you've already hit all the "biggest" movies, but occasional "premium" DIFs like this could be an option.

However, avatar's idea is even better in my opinion. You have a pool of pre-funded DIF movie suggestions to choose from, so you aren't obligated to do a particular movie you don't want to, but if you do do it, you'll get that money amount from DIF members. I can definitely see people getting together and throwing significant money to get you guys to do particular movies.

Premium content - I like that idea. I would donate $50 towards a 'marathon', (defined as at least 3 x 2 hour movies or 2 x 3 hour movies, etc). Star Trek, Potter, Bond, Pirates, etc.

Also, there are still a few gaps in the big tentpole genre series to fill in (e.g. Transformers 3). And coming up in 2012 are some big ones to knock over: Prometheus, Avengers, TDKR, etc.

Or old genre classics (e.g. Total Recall 1990 - perhaps doubled with the 2012 version).  Or overlooked left-field movies e.g. Mr Nobody. Or TV episodes. Or shit movies for fun sneering e.g. AvP. Personally I'd like some historical epics be given the DiF treatment.

So there should be lots of Venn Diagram overlaps between DiF and  the forum, so that no one feels pressured commentating on or paying for stuff they're not interested in.

Just to put my cards on the table and some figures...

For individual movies, I would say $10 is a reasonable price. I'd pay $10 towards commentaries for King Kong 2005, Benjamin Button, Troy (Extended), Kingdom of Heaven (Extended), Gladiator, Alexander.

I'd pay $50 towards a 3 x King Kong treatment, Millennium Trilogy (extended), Pirates 2,3,4 and maybe even the Underworld series. There's probably many more options - I'd have to think about it if some business model became more concrete.

Does anyone else like the concept of original & remake as a double-feature?

Maybe it could be a good idea to expand the site a bit more, making it more of a general movie site with news, reviews, articles and such? This could potentially up the page views and would allow for more traditional ways of gaining some sort of profit. Us forum folk could even be 'hired' as staff writers so that there is not too much of an extra work load for the guys.

Another way could be to go the Video Copilot route and offer some free vfx/general filmmaking tutorials alongside with some paid packages.