Hi Casper and welcome to the forum.
You already have nice setup with lots of options.
You can easily ‘expand’ your BS4 with an Active Kit and some Beolabs.
To do that however, you will need to have a Masterlink cable running from the BS4 to the room, where you need ‘more sound’.
That would be a relatively cheap solution depending on which speakers you choose.
But it is not always possible and/or wished for to have this cable running through the apartment/house.
Maybe you could concider buying another Core (or an Essence MK2, if you can find one) and connect the speakers there.
Since you already know how to hande the NL devices, it should be no problem for you to set up.
Or you could simply buy another M5, a M3, A9 or any of the other new/newer devices.
As you probably already are aware of, you need a good wifi setup for these devices alternatively a network connection.
Lots of possibilities today of combining new and old.
MM