Definitely 'other', and the biggest impact is probably something the Developers can't cause, but can certainly hinder...
Knowing people who play and love the game, who can explain how things work and get people involved in the guilds, events, and general 'community', is probably the most effective option going to bring in new players.
Simple marketing would fail (especially when it's done by a company who does not grasp the concepts of the product at all), a series of pretty pictures and standard PR hype will fall apart as soon as people see the game in action - it's not as pretty (in what is now the conventional sense of what people expect to see on-screen), or simple to get going in, as almost all the other mmos out there
Dynamic anything works only if the players use it and take to it, and if it's made abuse-proof. All those options would be great to see happening in-game, but you won't draw people in by saying 'look, destructible terrain and town council elections'.
New client and engine ... hmm ... If you were making a brand new mmo, you wouldn't start from here - but we are here, and as was demonstrated the investment needed to get a client that people accept as a faithful evolution of the 'classic' is massive, and probably way beyond EA/Mythic/BioWare/whoever we are this week to commit. What this option basically ends up with is - do we make a new game with the 'history' of UO, or not. The answer seems to be consistently 'not'.
If the devs get things right, EA make small announcements rather than ridiculous PR claims, and they get people talking about the game in other forums around the internet, we will see people joining, because the enthusiasm of players will spark interest in looking at the game, or returning to it. But it all needs to be realistic... it's a game where you have so many options, things are so massively interrelated, and there are no 'sides' other than the ones you choose to make and play with, so it's a big culture shock to those experienced with and expecting the current style of mmo.
UO will only keep those players looking for something 'more' than is on offer in other games, and it's keeping players that generates revenue for EA and keeps life interesting on the servers. Whilst that's probably a big market, it's not a 'top twenty' size market, so the best way to get it growing is for people to find other like-minded folks through the web and persuade them to try - don't overhype it, warn what the initial experiences of UO may be, and stress the longer term potentials of playing.