With a game like that, if it's on desktop, there's really no way around the black bars. With the game at fullscreen/scale outer it would be like trying to watch a football game on a 60" tv, when your eyes are only a foot away from the screen.
However, on a mobile phone/tablet the screens are pretty close in relation to their screen size (not resolution). A galaxy tab 10 is huge, but still not that far from a normal mobile phone screen. Yes, your objects need to be designed to be able to scale up to that higher resolution without losing its sharpness, but it can still be done.
if your game's layout is 1280x720, i'm assuming your window is too. Make your window 1024x576 (that's still a 16:9 resolution) and this should give you enough room to display on a variety of devices. Phones AND tablets.
And what happens when new smart phones come out with a 4:3 aspect?
No problem. Your larger layout with the smaller window will cover them too.
You just need to do some homework to learn how to move those walls to the edge of the viewport when it changes from 16:9 to 4:3.
There are expressions you need....get the viewport size > set walls to viewportleft, viewportright, etc. etc.
That is rudimentary, and not at all the expression. It's been awhile since i had tyo know it. And haven't used it in a LONG time.
I am not experienced as a lot of folks on here, and they may know a better way to do it, but that's probably the way i would try to go.