There is no possibility of fusion becoming competitive. A fusion plant would cost at least 10x fission, and fission is itself very far from competitive. As costs for renewables continue falling, that target gets always further out of reach.
Centrifugal storage could be useful for a few seconds, perhaps to smooth over sudden spikes. But it has been well-understood for over a century, and not used yet, so that is unlikely to change.
Natural gas and other fossil fuels are the problem we are trying to solve, so they cannot count as part of a solution. For the present, NG must be used when renewables fall short, because any storage we built out now would have to be charged up from fossil fuels anyway, as renewables do not exceed demand yet.
Practical storage will turn out to be based on an eclectic mix of air pressure (underground or undersea), liquified nitrogen, chemical synthesis (hydrogen, ammonia), hydro (conventional pumped surface, and underground and undersea variations), plus some small amount of batteries (liquid metal, iron). The first few are attractive because they are compatible with existing gas furnaces. Hydro is attractive because it is very mature. Chemical synthesis is attractive because once local tankage is full, any surplus power generated can be turned into revenue, and any shortfall in storage can be made up by importing more.
One more possibility is subsea buoyancy; an air bladder or rigid air tank is drawn toward a pulley anchored to the sea floor, yielding power when it is allowed to rise again.