I was more thinking that my one voice would rather be trumped by the many other people who are also suggesting things for the library to buy. They aren't made of money, and must prioritize. They would, and probably should, purchase things that more people want.
It depends; if there are enough funds, *all* suggestions by patrons will likely be acquired (unless they would violate the library's collection development policy). For example, at the library I work at, we acquire about 500000 new titles per year (branches included), and out of these about 1% are suggested by patrons. Local needs are always prioritized in collection development (also to make sure we don't acquire a lot of material that will mostly gather dust on the shelves) and patrons who tell us what they want to read/watch/play/listen are signaling "This is a title that will check out" -- after all, we're developing the collection for the local community, not ourselves or to build an elitist "dream collection". This is also the reason why we have so many manga series -- teenagers and young adults are far more eager than adults in telling our library what they want to read (or, at least, manga fans are). Having said that, all libraries aim to build a comprehensive collection with diverse material. Even if hunting is the "big thing" locally, it doesn't mean the library will spend the majority of its resources on material about hunting; it will be emphasized to some degree, but any good suggestion will likely be purchased (if we only wanted to make the circulation numbers look as good as possible, the safest tactic around here would likely be to purchase thrillers and manga books only).
Naturally, if they don't have a lot of funds, or the community analysis shows there is no actual need for a RPG collection (e.g. only a couple of patrons who admit being role-players), it's likely that they won't start developing it (rather, they'll suggest getting RPGs as interlibrary loans). However, if there is a local RPG community (but who are shy about asking the librarians about RPGs), the local public library might just as well acquire less books about hunting, for example, and spend the money on RPGs.
4E would not be a bad choice either - expose new folks to the game for which they are most likely to be able to quickly and easily find the most other materials and players.
CoC... I'm not so sure there. Great game, but the Cthuloid Horror genre isn't widely recognized by people who aren't already exposed to gaming. I'd probably aim for a system that does genres that are currently pretty hot - I'd probably aim for a Supers game (Mutants and Masterminds, say). After that, something that does urban fantasy well.
It depends on a number of factors (e.g. collection development policy and local needs and whether you have Lovecraft novels in the collection or not), but I'd say CoC would be a good choice for a horror RPG; it's a classic that draws its origins from classical horror stories (which, as mentioned, are found in many libraries' collections). In general, I like systems that have been inspired by literature -- such purchases often"strenghten" the collection and circulation (as a non-native speaker, I can't find a better expression in English) and are also supported by the literacy sources the draw inspiration from (i.e. works of Lovecraft may inspire your CoC campaigns). For example, if you have 'Mouse Guard' graphic novels, it's not a bad idea to acquire the RPG, too -- maybe some patrons who read the comics will try the RPG, and vice versa? Also, it's not just new gamers I'm thinking of; maybe there are "veteran" role-players who just haven't been vocal about their needs, or after a few years these guys who started with 4E would like to try something totally different? Just as there should be games of a variety of genres (fantasy, perhaps, a bit more dominant than the others), I don't think a new collection should consist on "entry-level" systems only (besides, I personally think many indie RPGs -- at least mechanically if not thematically -- are better "entry-level" games than D&D).
Call of Cthulhu, Vampire, D&D, RuneQuest, Mutants & Masterminds, BESM, SW Saga, Serenity are probably the systems I would start a new RPG collection with... (note that I'm dropping these names as I'm going; if I *really* had to do something like this, I'd spend a bit more time in analyzing different factors).
