The multiplayer function has already been covered by several MODs of FNF. Multiplayer tools: FNF being an unfinished game, the truth is that it does not yet offer all the functionality it should.
Nowadays, it is difficult to find a good MOD that does not use one of the Best FNF Engines.
On our website there are hundreds of these MODs, and many of them have a very good quality.
If what you are looking for is to install new “playable” characters, which to replace by BF, you will have to download playable skins, while if you want to face new opponents, you will simply have to install MODs containing new weeks with their own characters.