I once read (can't find the source though) that "law" and "medical" StackExchange websites are not going to happen. I can't also imagine the SE team to create Q&A's on strictly crime topics, such as, say, murder. Is there a list of topics that shouldn't ever be considered here, or some other kind of guidelines?
I don't think that "law and medical sites are not going to happen" is an official position of Stack Exchange. There are two possible ways of interpreting such a statement, both of debatable validity. One is that these sites are far enough from the core user base of Stack Exchange that it's hard to see them attracting enough experts to make it through the Area 51 process. The other is that there are certain aspects of these professions that would somehow be incompatible with the SE format of Q&A. Neither of these would be a valid reason to close a proposal, but the latter could be a reason for closing a failed beta site.
As for your question, there is no exhaustive list of these topics, but there are guidelines. Generally speaking, (almost) anything that has the potential to produce an expert Q&A site is allowed. That means there has to be some group of experts on the topic you propose, and some body of knowledge those experts are acquainted with. What exactly constitutes an expert or a body of knowledge is pretty flexible and open to interpretation.
To be a bit more explicit, there are four reasons why proposals can be closed. Until recently both moderators and high rep users could vote to close; now it's restricted to moderators. In order of frequency (from most to least frequent) they are:
- Duplicate (of either a site or a proposal): When there's already another proposal/site where all questions could be asked.
- Not a viable proposal: When the proposed site doesn't fit into the SE model of Q&A. This is used for things which aren't aiming to be an expert Q&A site at all, like the various "Miscellaneous"/"Other" proposals that people make regularly. It's also used to close proposals which spend over 1 year in either definition or commitment phase.
- Poorly Defined: When it's hard to tell what exactly is being proposed. The proposal might use incoherent or very ambiguous language.
- Content Violation: If the proposal would likely violate the Stack Exchange Terms of Service or Content Policy. In practice this is quite rare. I've only seen one proposal which was correctly closed this way, specifically a proposal for a marijuana site. A couple others (e.g. Trolling) were incorrectly closed this way, but reopened by moderators later. As far as I can tell, this is the only reason why a particular topic wouldn't be allowed on SE despite being a potentially viable site.
So long as you avoid any of those, your proposal is probably reasonable.