Finally someone thinking along the same lines as me! An air-breathing Plasma engine would save a lot of propellant since you don't need to "Burn" fuel. You could use microwaves (like the Vasimr Plasma engine) to heat the incoming air to a high temperature plasma and magnets to aim the exhaust as well as keep it from frying the rest of the aircraft. You could use a large Fuel-cell system to power it all (probably Hydrogen-Oxygen or something) or later a Fusion reaction (like the Polywell Fusion reactor) . Other options could be laser plasma heating or even a nuclear reactor.
The benefit of plasma is that you would have no moving parts, especially if you have a ramjet intake instead of a turbofan intake! The magnets would do all the work so less maintenance to worry about. Plasma exhaust would also be much hotter, so you'd get a higher specific impulse combined with the huge amount of mass from the air would give a large thrust and incredible speed (if you could heat enough of it fast enough) at very high efficiency. Imagine the power of the space shuttle but in a craft the size of a private jet.
You could even create a small plasma field outside the ship controlled by magnets to keep the superheated air from causing friction and heat on the aircraft while it's flying. That could also be used to reduce re-entry heating as well so you wouldn't have to have tons of heat shield tiles like the space shuttle does.
If you carried a little extra propellant onboard then you even could travel into space and back.
Just a thought...
