Doom and FF are like bread and butter. Yes they can function without one another but they go great together. The recent Secret Wars at its heart is a Doom vs Reed story (an oversimplication but still). They don't even need to be enemies. There have been instances that they team up one of which was during Hickman's Future Foundation. Future Foundation doesn't necessarily fix the FF (I don't think there are any problems to begin with but that's just me) but it expands it. It's no longer 4 people but people from different backgrounds coming together to expand the boundries of science and solve the problems world is facing.