Came here expecting a train wreck, but was pleasantly surprised 
Tl;Dr I don’t there is going to be an issue with appropriating from DDA.
Brief history lesson. Project was started by whales (this is the one sticky point since whales is an alias, you might want to contact him directly if you are concerned about clearing rights to the game, but see below for my thoughts on why you don’t need to). Whales ceased active development, I started writing patches, whalescwas convinced by a third party (GlypgGryph) to release his code under an open license, i backported my changes onto his code as of when it incorporated the license agreement, implicitly accepting the license. Then I ran a campaign of a few months to track down previous contributors and convince them to explicitly accept the license (a moderately successful due diligence excersise, good enough for me at least). At the end of this process, I’m confident that we’re properly licensed under CC-BY-SA.
Now for why none if that matters.
What is covered by copyright, and therefore the license? Code and assets, that’s it. There’s no such thing as rights over “brain feel”. If you want to make a post-apocalyptic THING using the sensibilities of cataclysm (everything wants you dead, its complicated out here, little shit wants to kill you too, none of it makes sense anyway, pragmatism over style, relentless realism), great, but you don’t have to ask us about it, we don’t own our style, it’s too intangible.
If you want to use the name.
- Don’t, blizzard will eat your lunch.
- We haven’t registered the trademark anyway, see #1.
You are super wrong. A game company could do exactly this, and not even need to port it. The only constraint is that If they do make extensions, they’re still licensed under CC-BY-SA, so anyone they distribute it to can pass it on for free.
In fact the iPhone port does charge for it, but that’s fine, they put a lot of extra work into it.