Being as I have the absolutely real theory of everything, I have some idea of what a UFO is capable of, how it would need to work, what types of propulsion are possible. What shapes would seem the most likely to accommodate certain types of engines.
Firstly.. Invisibility. It is very likely that a Alien technology UFO would be invisible. There are two reasons. First.. a negative mass ship wouldn't collide so much with matter at the speeds required, and negative mass is invisible to us. Second just to cloak from observation. That's what we would do if we got to the position to visit an alien world.
There aren't that many ways to achieve faster than light travel. You can create a particle torch which shines out Dark Flow. Then get the ship into the Dark Flow.. speed up the dark flow, and accelerate whilst at the same time accelerate the Dark Flow. Best not to point the beam at your destination though, best to point it at a nearby moon, because you will make a crater.
Another way is to create a massive bow shock. This is really destructive, but compresses time, and shortens the distance to where you want to go.
And the last way that I can think of is not so fast, but generates unlimited energy, but is quite dangerous. You have to fold space time, but not shorten it like a worm hole, because I don't seem to have figured that out. But fold it to create an engine. You will get a vortex that propels you.. but you are limited to mass tolerances. So nowhere near the speed of light. A UFO using this technology would stand out a mile, because it would look peculiar, with a big disc of plasma in the middle.
Some combinations of these.. switch positive mass on behind you, switch negative mass in front of you, and you will be in some sort of space tidal wave. This is anti-gravity really.. you have switched gravity around, but also strengthened it.
Can't really think of any more ways apart from the obvious solar sails etc.
We can't do any of the UFO versions until science gets a grip on negative mass. And I haven't come up with a way to see it either. But the two slit experiment, neutrinos, quantum tubes of silicon, and Quantum Gold stand out for me as the most likely candidates to come up with the answers. Mirrors too.. possibly.