Fan projects based on existing games are always at risk from publishers and other trademark holders who might decide to take legal action, removing a piece of media from distribution, or taking other less punitive measures. There’s a line of thought I see circulating among game journos whenever a decent fan project starts to receive attention: “Writing about this is worthwhile because it is great, but we would be summoning the Eye of Sauron.”
DoomRL [official site], the Doom-themed roguelike, has had press attention for years now, but ZeniMax hadn’t made any legal demands in relation to the Doom trademark until late last night. Now they have, but it doesn’t look like the end of the world.