IC: Big Boss fight arena
In my opinion, 5e sounds pretty good, especially if you want to make it less crunchy and more about the writing/roleplaying of it all.
Though I tend to prefer Pathfinder for its crunchiness, something like boss battles seem far more suited to 5e, if only because there's more opportunities for one-liners instead of having players who throw flour at your invisible boss because yes, they bought 5 pounds of flour at level one for this exact situation.
I agree with @engine that there is a benefit to a lead up. The journey to a destination can be quite as fun, even if in quick summary, as the actual destination (and perhaps could affect the boss fights in some way? For example, PCs could choose to sneak up on a dragon and catch it unawares, but on its home turf, or they could publicly challenge it and fight on turf of their own choosing...Ah, I'm overthinking it :D)