This has been alluded to in other threads, but is now confirmed
in this press release about increasing domestic connectivity in Sweden...
Furthermore, SAS is in the process of establishing another lounge, located near the security checkpoint. The aim is to provide travelers with a lounge within short walking distance, regardless of where their flight departs at Arlanda. The doors are expected to open in the first quarter of 2025.
This is
probably going to be in the space formerly used by the AmEx lounge, on level 3 at the terminal-end of F pier.
The press release makes an interesting inference about the new configuration of ARN-T5. With the new consolidated security checkpoint at the far north/east end of terminal 5, all passengers now enter the airside part of the terminal adjacent to the F pier. There's now a lot more backtracking to do if your flight leaves from piers C, D or E.