Good idea, always easier said than done!
Not a subject to be treated lightly, so, with apologies as to length...
Most of my experience has been on RPoL and the currently defunct (but returning soon?) GoAnd boards. RPoL is written specifically for PbP RPG's, and so has many bells and whistles that are extremely useful and desirable, but not strictly "necessary"; GoAnd did just fine without many of those, and iirc used a (modified?) phpBB as its platform, as does this site. (GoAnd was sold to a group not interested in supporting the RPG aspect.)
There are many "policy" decisions that you need to make- questions of autonomy, moderation, GM/Player "rights", what sorts of stories one can tell (ie, do all stories/language require an "All Ages" rating, or can some mature games include a "Warning: Graphic Language and Violence", etc; I'm guessing you don't want to invite truly deviant stories- that opens an ugly door*.) How games are advertised, how Players are recruited/selected, and such are also considerations that should be addressed, but again, largely, any practical policy is fine so long as its clear and enforced/enforceable. In general, a forum for advertising, another for chat, and another for the games themselves would be a minimum, with additional forums depending on traffic load/demand. (I'd be happy to discuss such topics at more length, not to tell you what is "right/wrong", but share some issues I've seen raised elsewhere.)
(* Laws are different in every nation, and this is an international community; however, it's not a publically owned site, and community standards still apply. The best policies I've seen allow such to be "mentioned", but not dwelt upon. Nor can a story be based purely on such - for example, torture was a reality of this historical period, but Ars Magica is not a game "about" torture, and the sagas should not be either. Some players would abuse a policy any less stringent.)
Many PbP boards only allow the User Name to be used in posting- thus, if I were playing a Magi, a Companion, and 3 Grogs, they'd all be posted as "Cuchulainshound", and only in-text labeling could distinguish between them. RPoL takes a diff approach, allowing any number of "characters" to be owned within a single game by a User in that game; thus, each character gets their own name (and their own avatar/icon/picture, but that's another can of worms.) And likewise, the GM can create any number of distinct NPC's - it is far easier to read a thread when each character's post, PC or NPC, grog or mage, is made by that character, unique and distinctly so, and not all by the same controlling User Name.
Multiple threads are almost a necessity for any one game, and as such, a GM should be able to create threads in their own game as they need them. (Whether those threads can be further "nested" or not is a policy issue, and a matter of personal preference and style, but imo as a GM it'd be very useful.) RPoL allows "private" threads, which can only be viewed by certain "Groups" of players- the GM can decide which players can see which groups, and so on- this is a great option, but hardly a necessary one. PM's could suffice, but the GM should have the ability to control that.
And, as mentioned above, an ability to insert jpegs into threads for visual aids, battle maps, or just mood is a huge bonus- not strictly necessary, but so much better than not.
Likewise, a GM should have some Moderation abilities for the threads within their game. If a post is made that is unacceptable (as some players have been known to get badly confused, or just plain go off the deep end!), deleting or Editing a post before others respond is a nice option. If possible, GM's should be able to choose who can/can't post in their games, since uninvited posting is, well, just that.
(RPoL has a special PM link within games for requesting to speak with the GM for a given game, but that's necessary there since a User can create a pseudonym for their public "GM" persona (and all players) - there's no way to tell who's running a game, or in one, if they don't want you to..)
Lurking/reading by either non-players or visitors to this site is, I believe, a desirable thing, tho' if technically possible it could be made optional. Open threads achieve 3 important and desirable results: more community interaction, more general inspiration and impetus for tabletop games, and community moderation- anything truly objectionable is more likely to be noted and reported.
Overall, I think this could be a good thing.