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.
Before looking at the content of the letter, which the developer of DoomRL has shared online, it's important to mention Jupiter Hell. That's a "modern, turn-based sci-fi roguelike" inspired by the Doom roguelike and by Doom itself, and it's currently on Kickstarter. It's entirely possible that the existence of a commercial project with such direct ties to DoomRL is the reason for this attention from ZeniMax, and the release of a new DOOM this year might well have played a part as well.
If you visit the DoomRL page right now (and it may change sooner rather than later), the top post is a link to the Jupiter Hell Kickstarter and the graphic at the top of the page is a Doom logo in the traditional Doom colours. The source of inspiration is not and has never been hidden - "a fast and furious coffee-break Roguelike game, that is heavily inspired by the popular FPS game Doom by ID Software" - but the link to a commercial spiritual successor is relatively new.
In their letter, ZeniMax ask for the removal of "all ZeniMax trademarks from meta tags, keywords, media, and other visible or concealed text that are connected to [the] website". I'm no legal expert, but that certainly isn't the harshest warning shot I've ever seen. There's no demand to remove the game itself, which does use enemy names and other elements of the game, though not assets, and Jupiter Hell isn't directly mentioned. The name Doom is starred out (D**m) on the Kickstarter page already.
You can see the letter in full here. The key to ZeniMax's argument is that "unauthorized use of ZeniMax's intellectual property falsely suggests ZeniMax's sponsorship or endorsement of your website". The reason the company is involved in this is due to their ownership of Bethesda and many associated studios, including id Software. I don't expect corporate entities to sponsor or endorse anything unless they're getting something in return, but it's nice to see that at least one person closely connected to Doom does support Jupiter Hell, which looks fab.
It seems unlikely that DoomRL is about to be purged from the internet, and while changing the name isn't a case of simply applying some TipEx to the internet, DeimosRL might work in a pinch. And maybe TristramRL and LV-426RL (rolls off the tongue) for a couple of those other projects.