If they would have a girl which is almost same age as Malcolm and Reese, I picture her as clever little brat, who always plays tricks on her brothers. The boys would build a plot to take her down, but she would always outsmart them.
That's because she is a Wilkerson woman, like Lois. She would always win. And, what tops it all is that she has many friends who like her; other girls and boys, a fact that makes her brothers even more jealous.
She would be Lois' favourite child, whereas Hal would stick to the boys. Hal would always say the boys are right, Lois would rely on her reasonable daughter. So there is a constant rivality considering Lois and Hal too.
Can you imagine that? Would be interesting to see such a constellation.