There are some interesting maps and quite a few weapon variants in BF1's single player that aren't playable in multiplayer, and it's a real shame because these features look excellent. A great example is "The Runner," which features a small trench complex on Cape Helles not seen in the Turning Tides map, a very detailed town area, and a large fortress similar to Fao. Some examples of weapon variants include a C96 with a suppressor, another C96 with Radium Sights, a C96 Carbine with a Marksman scope, a suppressed M1903 with a Marksman scope and bipod, a Gewehr M.95 with a Sniper scope, a Howdah "Sweeper" pistol that fires small shotgun shells, etc.
Frankly, I don't know why these aren't available as multiplayer features. While some of the maps would be small, they would make great TDM, Domination, and maybe even Shock Operation maps. The weapons are just new variants with their own strengths and weaknesses, so they should be balanced just fine.
Basically... Why not? A few tweaks, and we have some nice new BF1 multiplayer content. And yes, I still maintain that DICE needs to work on fan retention, especially after the BFV fiasco.