This has been discussed before, but it's useful to discuss once in a while to see what has changed with the various nerfs, buffs, and new things.
The main benefits that marines have are long range weaponry and the ability to call in strikes in places that they can't see.
So generally you want to have open areas with clear fire lines and as little cover for the xeno scum as possible.
A lot of times it's also good to tear down walls that might give cover to any attacking xenos, because the further it is between cover and the front line the more bullets they have to soak before attacking the barricades.
So any other observations and opinions on what makes a good FoB nowadays?