I think black or brown is a personal style choice. I usually go for black but something like a very dark brown textured goatskin could be really nice, eg
I actually think its gold plated - the old man was always a bit of a cheapskate ;-)
That strap is very nice and getting a Longines buckle would be a nice touch, god knows what happened to the original.
Really lovely, is it solid or gold cap?
I think black or brown is a personal style choice. I usually go for black but something like a very dark brown textured goatskin could be really nice, eg
https://thestraptailor.com/products/alran-goatskin-vintage-minimal-stitch-chocolate-brown
(But without the contrast stitches.)
Black Bough have some OEM gold plated Longines buckles by the way: https://www.blackbough.co.uk/product/longines-wristwatch-strap-buckles/