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99% of fangames are not covered under fair use, but you're never going to run into legal issues if you aren't selling because as Nifflas stated, it's simply not worth it for the parent company. I always recommend people work on original games because it's much more worth it to create your own project rather than another fangame, but nobody actually cares and nobody has the right to tell them they can't.
Those fangames aren't covered by fair use because they are named after existing trademarks and uses assets from other games. Yet, the Castlevania fan games that does all this aren't taken down. In this case, we're talking a game that doesn't use Castlevania assets or infringe on its trademarks. And if somehow "Hasslevania" gets into trouble for having a name similar to "Castlevania" (which will never happen), I'm sure fair use would cover that.
And on topic, check out the Mumble Indie Bungle which is lovely.