The existence of easier Angband variants doesn't take away from the original, IMO. The main concerns I'd have are that people would make it too easy (winning loses meaning) and be able to reverse engineer savefiles too easily; that people would muck with the story; and that all the SECRETS and code would be revealed...

I'd say that the first thing isn't a huge loss (cheating is always possible anyway, just hard); the second one is potentially cool (it might not all suck, and a mod with more options would have a lot of neat stuff with it), and the third...well, I don't care, I wanna see it!

Yeah you'd get mods pretty quick that totally change the world, just using the combat engine. And mods that change the story and muck with the cosmology. Is that the worst thing that could happen to a project? Becoming a base for other, potentially also cool stories? There's going to be an early flood of simple hacks and cheats, but you'll also get some -really- advanced stuff. And you'll draw back old players who stopped playing, too.

I'd say use a noncommercial license that keeps ownership of the original, doesn't require modders to release their source, but that requires a prominent link to the original ADOM on the title screen. (I say don't make other people release their source because...well, it will have taken other players many many years to ever get the whole story, you might as well let modders have the same satisfaction...)