We had been considering buying Merchants & Marauders, but it has an advertised playing time of 3 hours, and reports of taking even longer. So realistically, if we play it at all, it's not likely to be often enough to justify the price.
Tikal II: The Lost Temple was our other possibility; we need a few more voices in support of that before we take the plunge.
Oh, and this talk of big long games reminds me that I quite fancy Elder Sign -- it being similar to Arkham Horror, but shorter and faster:
Elder Sign is a fast-paced, cooperative dice game of supernatural intrigue for one to eight players by Richard Launius and Kevin Wilson, the designers of Arkham Horror. Players take the roles of investigators racing against time to stave off the imminent return of the Ancient One. Armed with tools, allies, and occult knowledge, investigators must put their sanity and stamina to the test as they adventure to locate Elder Signs, the eldritch symbols used to seal away the Ancient Ones and win the game.
To locate Elder Signs, investigators must successfully endure Adventures within the museum and its environs. A countdown mechanism makes an Ancient One appear if the investigators are not quick enough. The investigators must then battle the Ancient One. A clever and thematic dice mechanism pits their exploration against monsters and the sheer difficulty of staying sane and healthy, all within the standard game duration of one to two hours.