Tbh, that would really make the whole point One punch man over thought. Saitama was never struggle in any battle before. Well at least right battle. For example, the final alien boss in season 1 vs Saitama show Saitama was not able to finish him with one punch but it’s not even a serious punch. If Saitama actually fight seriously, no one can’t beat him.
Except he hit Boros with several punches before, they weren't serious, but that shows that it took more than one punch to kill him, which is "the whole point of one punch man" according to what you said. His whole point wasn't lost, and people didn't run away from the show because he didn't defeat boros in one single punch, he still beat him after the serious punch, and after hitting him many times before. People were even enjoying it because they thought Boros could force Saitama into revealing more of his power, which he did with the serious punch. That's the whole point, it'd be awesome if he showed all of his power, and found a need to gain even more.
I'm not caught up on the original source material, but I would hope that in the future this "god" character could be a serious threat to Saitama, it could give him a hard time, maybe even nearly killing him, and in the end he defeats it. Unlike the other villains though, killing it would be impossible, and one day it will return stronger than before. This would allow Saitama to strive for something, to finally find a purpose for his power, and to increase his training. I think it would motivate the other characters as well, since unless he literally became a god, eventually Saitama is going to die of old age, and someone has to become just as strong or stronger for when that foe returns.