Im no expert but the attempts were early enough to really have an impact in a sense that the sengoku period is likely prolonged.
 
According to Wikipedia, there were at least three attempts on Oda Nobunaga's life by ninjas.

What if one of these attempts had succeeded?

Okay, in any event, it is left to Nobutada to pick the reigns, the good news is most enemies are either pushover or too weak to fight him. The bad news even if Nobutada is married to a daughter of Takeda Shingen at this time, it really doesn't stop a man like Shingen from attacking him. However, Shingen's appointed regent Katusyori and the man who would be Takeda Nobukatsu could be open to peace.

If Nobutada is not as ruthless as his he may keep Akechi Mitsuhide's loyalty, but Nobutada is still surrounded by capable men and is somewhat competent himself. So he might be able to succeed where his father failed, especially if ironically enough this turns out to be a fortunate series of events.
 
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