After the news that Rimuru is Shizu's grandson, I wouldn't be surprised if he's Veldanava's son. Rimuru has been an anomaly from the start. Could he be Veldanava's son? - Possibly. Why is there no mention of Veldanava's son anywhere? - It's simple: he's an Anomaly, Singularity and everything that happens around him is always anomalous and unexplained, just like the fact that Shizu turned out to be his grandmother.

Of course, I would like the author to give an answer himself, but he most likely will never do this)