Preventing entry into boss arenas while new players are in cutscene would definitely help. Locking new players out of boss battles because they are actually trying to experience the game narrative is shameful, and unfortunately extremely common.

Some people hate cutscenes, or have seen them many times before, but that doesn't justify destroying the game narrative for new players by forcing them to either skip cutscenes or miss out on boss battles.

Many people love the story aspect of Final Fantasy - for those players, watching the cutscenes in the inn (or battles on twitch/youtube) is nothing like actually experiencing them in context, without (yet) knowing how it's going to end!

(Guildhests might already have a system in place where players are locked in a blue circle while new players are watching the cutscenes?)

Item level sync would also help reduce the incentive for overgeared players to lock the healers (at least) out of the boss arena.