Some random thoughts to take or leave. These apply to any timezone as I haven’t personally seen much evidence of a different culture depending on whether a server is hosted in North America or Europe:
i) Encourage players from less well catered for timezones to post their likely play times on their forum signatures. DMs and other players can’t work out when a given player is likely to log on by consulting a crystal ball.
ii) If there are only two or three of you online (irrespective of timezone) and another player logs on, consider how you can accommodate them if you aren’t in the middle of something major at a far flung corner of the server. It is extra important to be sociable and inclusive OOC when player numbers are low, for example, if you are playing at a less common timezone. If you don’t go out of your way to interact and encourage new players, chances are your server will gradually become less and less active.
iii) Where possible, get calendar functions on your forums to encourage players to organise pre-arranged sessions. Particularly at weekends, this will give players from rarer timezones a better chance of getting interaction with players from the main timezones.
iv) Provide even limited DM support to player instigated pre-arranged sessions where possible. It is a pity, but generally players seem to have no interest in such sessions unless there is an OOC motivation for them to do so. Even something like an RP based item in a loot drop, or an unexpected monster spawn would take very little effort but can encourage people to attend pre-arranged sessions in the future.
v) If you have a trusted player (particularly in a timezone with no DM coverage), consider giving them some tools to aid the enjoyment of others (e.g. some funds or items on the condition that they be used as RP incentives/props to provide motivation for other characters). Sure, there is potential for abuse, but there are ways of getting around that. A good player with some resources can be almost as positive for a server as a good player with access to the DM client. Alas, the most RP focussed players are often the players that have the poorest characters as they spend less time going from dungeon to dungeon. It is hard for the poorest characters to provide RP incentives to richer, more powerful characters. This could be used in tandem with iv) above.
vi) DM coverage is important, not just to inject a degree of unpredictability in the story, but for more basic things like freeing players who are stuck in the scenery, greet newcomers, make sure rules are being observed and standards maintained etc.
vii) Consider temporary/junior DM slots to players in different timezones if you are reluctant to hand over the keys to the car. Or guest DM slots for more trusted longstanding players to run a one-off campaign etc.
viii) If your server has XP penalties for soloing or being in a small party (a good thing in my book for RP servers to encourage partying and interaction), consider how to make this fairer in a timezone with very few players. One particularly elegant solution I saw was a place that withheld the XP penalty for soloing until the server had a certain number of players online.
ix) Get your server listed in
this thread so people can quickly check if your server is active at certain times.