I wouldn't be at all surprised if this has been pointed out before, so apologies if I'm being slow.
I've been watching all of Dragonball from the start with my son, and noticed a lot of similarities. Could have been a source of inspiration for Kirkman?
Saiyans (Viltrumites) are super-powered aliens who conquer the universe by sending out 'sleeper' individuals to soften up and prepare planets one by one. Goku (Omniman) is assigned planet Earth and sent there, where he has a son by a human wife. Goku and his son (Invincible) reject their Saiyan heritage, having been raised as human. The Saiyans' home planet Vegeta (Viltrum) is destroyed leaving only a handful of their race.
Add to that the fact that Allen becomes stronger every time he is beaten close to death, which is also true of Saiyans.
Of course a lot of that (super alien from destroyed home world becomes protector of Earth) is straight from Superman, but it's got the added Dragonball details don't you think?