I was really thinking about something new when I asked what was meant by "original". So here is some inspiration....
Something 'original' could be that you see the ship from above and it starts as a horizontal scroller but at some point in the level you hold space or button2 and can(in fact have to) rotate the ship because it becomes a vertical scroller.
Maybe even some diagonal parts and the ship is rotated by the game itself.
I bet it has been done before but that would be so cool.
I hate for example the unidirectional flight level in Turrican 2, for the fact that I can not
rotate the ship. It would be so much cooler if we could.
But the idea of having both H+V scrolling in a shmup was great.
And to be honest I can't rememer any other game using that method atm.
There are some that come to my mind with twinstick/unidirectional shooting,though.
That brings up another idea. A two player mode where one is in control of the ship and the second player can control a turret(upgradeable) and some secondary weapon like homing missiles which can be collected during the game by player one. They could be stored in an item database and the second player can select them via F-keys depending on the enemy.
Different collected mainweapons could be selected via the number keys.
I think there are enough one direction button mashers with masses of enemies out there. So I would really appreciate something with a unique feature in the shooter gengre. The shop system mentioned by Mrs Beanbag would make the game more interesting,too.