[quote=@KabenSaal]
'Kenpachi (剣八, Sword-Eight) is the title that is only given to the strongest Shinigami' It's not necceserily 11th Captain, just the stronget Shinigami. So theoretically, Mirja would be Kenpachi, due to her super-dense body making her omgwftbbq strong.
[/quote]

Just... Just no.

Also the title has traditionally been held by the current 11th captain, as they are recognized as the "strongest" in swordsmanship. Besides, if the title were realistically held by the "strongest" then Yammamoto would have been Kenpachi from day 1 and the title never would have passed on to anybody. And Unohana, the "first Kenpachi," was clearly still far [i]far[/i] superior to Zaraki Kenpachi before she trained him, but passed the title on by choice.