where people are already located
The wiki list of European meetup groups, filtered by "at least once in a month" (mentioned explicitly in the group description), gives us the list of possible candidates:
Note that some of the information may be out of date, the wiki page does not mention the average number of people attending the meetups, etc. (Also, it's not just the number of members that matter, but how seriously they take the topic, etc.) So we can use this as an "upper bound" on places where enough active European LW-ers live.
As the next step I would recommend looking at what the local groups have already done, using the "past behavior is the best predictor of future behavior" heuristic. Berlin seems like an obvious winner; they organized the 2014 and 2015 European LW Community Weekends.
In short, I'm wondering what place/group/organisation/activity could do for rationality in Europe what Berkeley does for rationality in the US?
Soon, we'll have LWCW in Berlin, which I hope will be an occasion to do some networking among people who think seriously about developing rationality communities. But in the meantime, let's do some brainstorming.
Important note: in comments to this post, please use only consequentialist language. For example, say "If we decided for the base to be on Malta, then X would happen" instead of "I think it should be in Malta, because..."
Of course, share anything else that you think is relevant to the topic.
Also, see you all in Berlin :)