First of all, I'm biased against guilds. Not only were they a total failure, they ruined any chance of WoE becoming a reality, something I was very passionate about.
I'd like to second that, mostly because of the last part.
But the way I see it, before anything, it's crucial to decide on a "clear purpose" for these team-ups to exist. There were many great ideas so far for guilds, but my main concern with most of them that they're almost exclusively based on sporadic events across the forum. And most probably, 'deadspots' between these events would drive a great portion of the non-forum-regular/newer guildmembers inactive. And even then, a simple guildpoint system is not something that you can keep yourself hyped about for long terms, while also leaves very small room for power struggle after a certain game time.
I think Guilds - to be successful - would need a dedicated "gameboard" or 'forum space' besides the possible links with existing events, where teams can continuously compete against others for whatever goals or prizes. And that's where WoE comes in: If the energy to
create Guilds back then would've been poured into simply
resuming WoE (even on it's most basic alpha/beta level); and
then introducing Guilds on top of all.. would've been simply epic.
So.. why not try the other way around?