I also use Mate on FreeBSD as my main desktop environment. Recently, I've even been thinking about what could replace it. The only alternative is the gnome-flashback mode/project, which is visually similar to MATE but based on the newer GTK.
I've tried this environment a few times on Linux.
Not for long, as I always returned to Mate or XFCE.
On FreeBSD, there's supposedly
root@freebsd /home/yampress # pkg search gnome |grep flash
gnome-flashback-3.58.0 GNOME Flashback session and helper applications
Unfortunately, I haven't tested if it works at all.
Has anyone tried to install/run/use this on FreeBSD?