I have no special insider knowledge. My impression, which I will gladly have corrected by those who know more, is that

• MIRI (formerly SIAI) was founded in Berkeley because that's where Eliezer was.
• Most of the rationalists in Berkeley are not MIRI employees.
• Most of the rationalists in Berkeley did not move to Berkeley because of MIRI or Eliezer or other rationalists.

But, again, this is vague impressions and guesswork and assumptions rather than actual knowledge. So let's assume for a moment that I'm entirely wrong and the Berkeley rationalist community is a consequence of MIRI. MIRI was founded about 16 years ago, and I think it's only in the last few years that the Berkeley rationalist community has been a big thing. That would suggest that the "build a rationalist community by starting an institution there" strategy takes 10 years or so to work.

If so, then good places to consider might be places that already have kinda-MIRI-like institutions. Perhaps Oxford (home of the Future of Humanity Institute, and also of Giving What We Can if you're the EA sort of rationalist) and to a lesser extent Cambridge (home of the Centre for the Study of Existential Risk). I think the FHI and the CSER are the nearest non-MIRI things to MIRI.

Btw I think the consequentialist version of your question is "What happens if the EU rationality community centres its efforts around a research topic (with the question being open to different consequences from different topics)".

Yes, Berlin is the most obvious candidate at this point, and it seems strong on most fronts except climate and prices.

Prices aren't bad compared to other big cities.

You would need around 3,087.02€ (2,438.53£) in Berlin to maintain the same standard of life that you can have with 4,500.00£ in London

You would need around 3,073.39€ (2,427.76£) in Berlin to maintain the same standard of life that you can have with 3,600.00£ in Oxford

You would need around 3,084.93€ in Berlin to maintain the same standard of life that you can have with 3,400.00€ in Vienna

You would need around 3,093.66€ (3,484.64$) in Berlin to maintain the same standard of life that you can have with 6,000.00$ in Berkeley, CA

You would need around 3,077.66€ (3,342.51Fr.) in Berlin to maintain the same standard of life that you can have with 7,300.00Fr. in Zurich

It's not anymore the price level it had when Tim Ferriss wrote the 4HW and recommended digital nomads to move to Berlin to have it super cheap, but it's still relatively cheap.

Which city do you consider suitable that is cheaper than Berlin?

In most cities I think you couldn't run an event like the LWCW for 150€.

I think that "not be too far from any other country" would lead to a more open community with more fluent membership status. People who feel vaguely connected to a community will go to events which are closeby, but are less likely to attend if they have to fly (and consequently have to plan much longer in advance). At least I'm willing to travel much longer in a train – where I can do work – if I can avoid flying that way, which seems to consist solely of controlls and queueing.

Have you added the important note after I made the comment or I just missed it totally? Anyway:

If the European Base is going to be Berlin, members of a larger group of nations will be able to attend, due to the position being almost central from any other European countries. The community that is going to be developed there, as an entity nested inside the German government / culture / etc., will benefit from less ostracism and a wider acceptance even in its weirder facets (cryonics / poly...) thus enabling a better freedom of movement / speech / publications, etc.