I just thought of something. Warning this requires a bit of math.
Kenshin was born in 1849. (http://en.wikipedia.org/wiki/Himura_Kenshin
Meiji Restoration = 1868, Kenshin is 18/19 years old.
10 years later, start of series = 1878. Kenshin is 28, based on misc sources.
5 years later = 1883 (http://en.wikipedia.org/wiki/Himura_Kenshin#Plot_overview
), Kenshin and Kaoru are married and Kenji is about one (born in 1882, maybe)
1904, Russo-Japanese war breaks out, Kenji would be about 22.
What I'm trying to say is I've been fantasizing, for lack of a better word, Kenji's adventures during this war. Why? Because in 1873, Japan instituted a nationwide conscription mandating every male serve in the Imperial armed forces for 3 years. Kenji would be at least 21 and thus conscripted. Not something Watsuki would've done but would be cool, if you ask me. What would Kenshin think about it? Remember, he's not dead according to Watsuki.
tl;dr Watsuki could've made a sequel with Kenji going to war.