As has been pointed out to me at least twice so far (and several times in the comments of the following Newsarama article), one can speculate that based on these comments from Dan DiDio at Newsarama (discussing Superman and "Action Comics"):
DD: ... there are so many fabulous twists and turns along the way, Geoff has done a great job of laying out basically, a blueprint for the Superman books that takes us through to the end of 2010, and it’s built around New Krypton, and quite honestly, the status quo changes every few months. That’s what keeps it exciting and interesting. Immediately, for example, in the month of March, there will be changes to Action Comics in regards to who the star of Action is, and you’ll see also when we bring back the title of one of the most beloved older DC series and present it in a new light as well.
... that we might see the return of "Adventure Comics". Historically, "Adventure Comics" has been an anthology title, starring Sandman, Manhunter, Starman, Superboy, Green Arrow, Johnny Quick, Aquaman, the Bizarros, Supergirl, Spectre, Black Orchid, Starman (Prince Gavyn), Plastic Man, Dial H for Hero, and the JSA over the years. It was the second title published by what later became DC Comics, starting in 1935. Oh, and of course, the Legion starred there for a while, too. DC has only four titles surviving with original numbering that started in the Golden Age ("Action Comics", "Detective Comics", "Batman", and "Superman"), and even though the series ended in 1983 in digest mode, it's still the 5th highest numbered DC title.
So let's assume that DiDio is indeed talking about a new "Adventure Comics". Do you restart it at #1, the easy thing to do, so that new readers can jump on a series with an issue 1? Or do you pick up from the old numbering, honoring the Golden Age legacy? And if so, what do you pick it up with - could the old series be considered to have ended with issue #478, after which it became "Adventure Comics Presents Dial H for Hero"; or with #490, after which it became "Adventure Comics Digest"; or with #503, after which there were no more issues?
And if you keep it as an anthology, as Jim suggested in an earlier comment, you could have the newly reintroduced post-Crisis Legion as the main feature and any number of other Legions (in particular, the post-ZH reboot and the current version) as a backup. I don't think the title would support two regular-length features in a double-size issue.
I got an email a couple weeks ago from Jason, one of my readers (who is the owner of the Legion_3008 Yahoo group, and formerly the Legion_3004 group). He had sent an email to Dan Didio expressing his displeasure at DC cancelling the Legion right at the end of "Legion of 3 Worlds", not a great way to celebrate the Legion's 50th anniversary. Dan replied (and I'm slightly paraphrasing, since I didn't ask for Dan's permission to reprint his email) that we should stay tuned, DC really does have plans to return the Legion to its former glory.
Which, of course, should really be obvious. They didn't hire Jim Shooter for nothing (though it didn't end the way everyone wanted), and they didn't build a miniseries written by one of DC's top writers, drawn by one of comics' top artists, and tied to DC's important crossover series to have the team/concept just sit there waiting for someone to come up with something. I have no idea who might be involved with the theoretical new title from a creative standpoint, but I'd bet that Geoff Johns is somehow involved.