You just cannot fight an army containing three dragons. The story of the field of fire (http://awoiaf.westeros.org/index.php/Field_of_Fire, no spoilers) has proven that in many ways. Jon might end up as Lord of Winterfell and warden of the north, not more if Dany is just marching through (what is hopefully not going to happen)
Then why did Dany need the Unsullied at all?
Anyway, this doesn't address all the reasons I've provided for why they may not fight each other at all.
Because the dragons weren't big enough!? Now she doesn't need them for battle anymore, just as cityguard.
Now to your reasons:
- Jon is actually [probably] a Targaryen. This could come to light in various ways, e.g. Dany gets another vision, or Jon demonstrates the fireproof nature that their family seems to have in the show. --> Might happen
- Dany recognizes that the sins of the father aren't the sins of the son. --> Might happen, still no reason to give half of your kingdom away if you own an unbeatable force!
- The White Walker's present a major outside threat. --> Definetly, but i hope the typical "joining each other to beat the threat and live a happy life afterwards" theme is not going to be part of the series.
- They're both against Cersei. Enemy of my enemy. --> Cersei is insanely weak and there is no need for an alliance (cf. page 27)
- Dany may sympathize with Sansa as another woman making her way through a patriarchal society. --> Might happen, still no reason to give half your kingdom away.
- Tyrion is reasonable and sympathetic to both Jon and Stansa. --> Might happen, still no reason to give half your kingdom away.
- Dany and Jon may also be tied together by prophecy. --> Might be the case
- It's winter. It's probably a bad idea to start a war in the North during winter --> Would be a short war (cf. "Field of fire" lasted one day)