As Sweeney claimed, the place where all these different threads of development come together is Unreal Engine 6. In short, the aim for Unreal Engine 6 is to bring the best of both worlds together. It will offer much easier gameplay programming for both the Fortnite community and developers. Sweeney also stated that UE6 will offer more scalability to large scale simulations of all sorts. Plus, it will offer greater ease of use. This means that studios will be able to hire programmers who are familiar with and experienced with the engine.
Moreover, UE6 will offer devs the full development capabilities. Thus, they will be able to build their game once and ship it anywhere. Moreover, standalone games will be able to access and use all the items in Fortnite. Similarly, UE6 assets from standalone games will be compatible with Fortnite (so that you can import and use them in there).