Fanon of the origins of Luna and Celestia has taken several forms but a popular theory is that they are the daughters of more powerful pony gods and goddesses that have roamed the heavens creating stars and planets-The very expressions of Matter and Void, to use the Atomist philosophy school concept. That is, one parent represented matter (and thus energy, gravity, etc) of the entire universe and the other represented the darkness and emptiness between various kinds of matter, from the spaces between the quanta of matter/energy to the voids between galaxies.

I tend to use Orbash of Dark and Tau Sunflare of Light for their names. Orbash, in a nod to Kingdom Hearts'
Yensid ("Disney" spelled backwards) is "Hasbro" backwards. Tau Sunflare, on the other hoof, is an anagram of Laura Faust (which seems only appropriate). But my idea only marginally concerns them.

It's been established that other alicorns and alicorn-like beings exist thanks to Princess Cadence and Queen Chrysalis's appearances. We don't know if the latter is just a twisted abomination of one or if as a changeling she just takes that default form. However, it does grant us an excuse to have other alicorns as characters, and even another sovereign of another world. In this case, Mars.

Prince Ares of Mars, as I see him, would have been a far different ruler from his sisters. Mars in this universe would be more akin to the Mars of pulp fiction-A kind of great Space Montana/Middle East, an ancient world where empires rose and fell across the vast deserts, plains, mountains and canyons. The native ponies may have greatly resembled those of Equestria, or they may have even been far different (green ponies with little antenna makes for a fun mental image). It would have been brutal, with wars for dominance stretching across thousands of years. A beautiful, terrible and dangerous world.

And long ago, it fell. Perhaps to Queen Chrysalis and her changelings, invading from the sky and feasting on the love that planet held. Perhaps to Discord who caused a bit of chaos before Ares' sisters could put an end to him. Perhaps something else. Whatever it was, Mars has been dormant and silent for at least as long as Nightmare Moon's first appearance.

But, as tends to happen in Equestria, ancient powers stir again on the Red Planet, and adventure and danger awaits.

Your ideas, ladies and gentlemen?

...That sounds soooo awesome!

I say, splendid