My situation is very similar to the one described by Nander previously but with a few specific differences.

(1) What is your home airport?
Stockholm, ARN

(2) What types of fares do you usually buy?
Intra-Europe Y, Transatlantic W (with point-based upgrades to J)

(3) How many miles do you usually fly each year?
At least 25k-50k

(4) Do you have any kind of status at present? What is it?
SAS Eurobonus Gold (will be lifetime in April 2025, so I have lots of experience within *A, but SAS is switching to SkyTeam at the end of August)

(5) What is most important to you in a FFP? (Frequent upgrades on travel, priority services when flying the airline, good award redemption rates, free lounge access, etc.)
The #1 priority is the ability and ease of retaining fast-track security and lounge access, whether through Star Gold or other means.
Ideally, I want a scheme where I can earn status no matter which *airline I'm flying on (including no requirement that X number of flights per year are on a specific carrier).

(6) Which routes do you fly most often? (US Domestic, Transpacific, etc.)
Intra-Europe (primarily Spain, Italy, and Portugal)
Inter-continental (primarily transatlantic to USA)

(7) Preferred Airlines
I'm fairly agnostic. Because SK has historically been within *A, a significant share has been LH group and TAP (in addition to SK) for intra-Europe and UA and AS for connecting travel in North America. With SK switching to SkyTeam, I'll likely fly more Delta/KLM to PDX and European destinations as appropriate.

(8) Do you travel for work or pleasure? (Can you choose your airline and class of service?)
It's a pretty even split. I can choose airlines, subject to there not being a significant price differential. For business travel, I always choose fully refundable tickets and premium economy for anything > 4 hours.