What it sorely lacks is any sort of graphic interface. Nethack is a text-only game that has only relatively recently even allowed for colour. There have been many attempts to add isometric views, tiled maps and the like but they have all failed due to the clunkiness of the resulting interface, and the fact that in some pretty deep parts of the code, it really thinks that humanoids look like the letter
Nethack has long been an open-source project, although any spelunking through the actual code base will rapidly reveal that multiple generations of coders over the last 30 years have rendered its internal structure to be well-nigh incomprehensible.
Now, a game with the opposite set of problems is Quake. There was a game with wonderfully interactive 3D graphics, a fully immersive fantasy experience, but fixed levels and almost zero replayability. Still, when it came out in 1996, it was a sensation. The Quake game engine has long since been released under the GPL and has been updated and tweaked in various different ways in a huge variety of open source projects. There are now a fair number of other open source game engines which one could use instead, such as Cube, Saurbraten and Crystal Space, but since Quake was the engine of choice when I first came up with this idea years ago, I'll continue to use it as the suggested basis. In practice I'd spend a fair amount of time assessing the various alternative before making a choice.
At some point it occured to me that it would be an interesting experience to marry Quake and Hack together into a sort of 'Quack' game. Steal the dungeon generation as well as the vast sets of items, monsters and interactions from Nethack, and use the real-time play and immersive 3D effects of Quake to interact with the results.
Now, this wouldn't be a trivial operation. Nethack has been tweaked so much that one would be better off taking one of the various data-dumper programs that have been written for Nethack and using it to extract all relevant datasets and throwing most of the code away than to try to actually port it directly. Still there is a treasure-trove of game design there that can be salvaged.
There would also have to be some careful user interface design. 3D worlds practically cry out for real-time play but Nethack was always more of a thinking game than a reaction game. One would have to make it practical to scout out areas without alerting monsters, pause the game when necessary and generally make it possible to think ones way out of trouble, rather than try to hack and slash through it.
So, the work would not be trivial, but I think it would be very rewarding and since most of the design work would already be done, the job would be a "simple matter of coding". If done right I predict the results would be very popular, and quite lucrative to produce due to possible derivatives, even if one ended up giving it away (after all, it would be derived from open source products).