Roleplaying guilds may be themed in different ways, with their members taking part in the guild as an in-character organization. Some of these themes include (The recommendations are not the final word. Any race you want can join):
Military - Knightly orders, mercenary guilds and militias. These organizations are focused on combat and warfare (PVP). Orcs and Humans generally run these guilds. Suggested Classes: Warrior, Rogue and Paladin. Race: Orc, Humans and Trolls
Religious - Monastic orders, priesthoods, and similar. If the guild serves a certain god, goddess, or other "good" being then this is their category. Suggested Classes: Priest, Druid, Paladin. Races: Human, Tauren, Draenei and Night Elf.
Cult - Similar to the Religious type, Cult guilds are usually much smaller, focusing on evil beings. Rather than being a part of an organized religion, they are a cult that may hold similar beliefs to other cults that worship the same beings, yet they tend not to retain any steady ties between themselves. Suggested Classes: Warlock, Death Knight, Priest. Races: Blood Elf, Forsaken.
Magic - Magical colleges, Magical Orders, and similar. This is one of the rarer types of rp guilds. Subsections of the Kirin Tor and other magic-oriented guilds could easily fall into this category. Suggested Classes: Mage, Priest, Paladin, Druid and Warlock. Races: Human, Gnome, Draenei, Blood Elf and Forsaken.
Explorer - Seekers of knowledge, in what ever form. Whether it be of the land, of ancient lore and artifacts, what ever, it falls into this category. Suggested Classes: Hunter, Warrior, Rogue. Races: Dwarf, Orc and Human.
Criminal - This is the broadest category. Any guild that participates in things that could be classified as a theives or an assassin's guild. Suggested Classes: Rogue, Warlock, Warrior. Races: Human, Orc and Dwarf.
Mercantile - If your guild is a company similar to the goblin Cartels, or anything resembling them, it falls into this category. Mainly this guild consists of people hunting enemies to find rare drops, and selling them on the auction house for either a fair price, or to scam lower level players. Suggested Classes: Any. Races: Any
Settlement/Clan - One of the more common types of rp guild. They focus around a single "Family" or "Clan" of players who are related to each other by blood or by living in an in game settlement. This can overlap with Mercantile RP if a guild takes over, for example, a tavern. The important thing is that they find a relationship through where they live and work, or through familial connections. Suggested Classes: Any. Races: Any.
These different categories can overlap a lot, for example, an order of paladins who serve the Naruu might be seen as a Religious Military RP guild.