Part of the problem for new start-ups nowadays is the accessibility of data.What used to quite opaque is now quite obvious and decent carriers will adapt to counter potential opportunities unless they are prohibitively costly. That's not to say that opportunities won't be out there but they are a lot harder to find in todays environment.
Given the amount of greenwashing going on in the airline industry, a truly green airline starting from scratch could get a jump start on traditional (in its widest sense) airlines if they could find a way to offer a hydrogen-based/electric hybrid option at scale. Other than that it's difficult to see what innovations are available for a new carrier to create a true USP given the amount of change over the last 25 years.
Having said that wasn't there some idiot back in the 1890's who said that "everything that is possible to invent has now been invented". There's always scope for a new way of looking at things.