well ... like i said its what you conclude after reading it all, and he also wasn't stated to be anything, but when you add every thing up the chances of him being a senju are more than anything else .... it was said that hashi's cells enhance izanagi and needed to carry all these sharingans but was never said that danzo needed it TO PREFORM IZANAGI thats why i think he was already a senju (izanagi requirements: senju DNA and a sharingan) witch could mean that he already had the requirements to preform izanagi (with one sharingan) but needed a way to boost it like using it 10 times and making it last longer, anyways its cool
I took this from the Narutopedia: "Izanagi can only be used by those with the genetic traits of the Sage of the Six Paths. The Uchiha, descended from the Sage, are able to perform Izanagi with their Sharingan. In exchange for the brief control of reality that it allows them, the Sharingan with which Izanagi is cast becomes blind. For this reason the Uchiha labelled it a kinjutsu. Danzō Shimura, in an effort to make practical use of the technique, had ten Sharingan embedded into his right arm.
To make full use of Izanagi, users must also have the genetic traits of the Senju, who are also descended from the Sage. Partially for this reason Danzō had some of Hashirama Senju's DNA transplanted into his arm, which extended the lifespan of each Sharingan's Izanagi to a minute, allowing him to use the technique for up to ten minutes in total, with breaks in between as he deemed necessary. However, because Danzō is not an Uchiha, his chakra levels drop substantially every time a new Sharingan is used to activate the technique. Obito also makes use of Hashirama's DNA but, in contrast to Danzō, he claims to have gained control over Hashirama's DNA and as such, boasts the completed form of Izanagi."
It only says he needed the Hashirama cells to prolong the use of Izanagi. I assume from the bolded part that he didn't have Senju DNA and that's why he let the cells of Hashirama be injected in his arm. Of course Hashirama's cells prolonged the use of Izanagi, but if he had Senju DNA, wouldn't that have been mentioned as well? I mean he would have been able to use Izanagi then, only without the prolonged use that Hashirama's cells provided him.
Anyway as long as it isn't explicitly stated that Danzo was a Senju, I just assume he wasn't ^^;