I'm just curious if the damage done to the cities during the war will be repaired. It's kinda depressing in Whiterun especially.

I'm not sure honestly, I hope so, Whiterun is my favourite city and seeing it in the state it is in makes me sad.

How many cities are damaged? How bad?

I totally agree, and not only reconstruction to the damaged cities but simple progressions to the most annoying dead ends ie. the Windhelm priestess keeps telling you "Diagonal cut on the left shoulder" even weeks later you finish the related quest. Faendal and Sven never come to engage with Camilla Valerius, Aventus Aretino mentions returning to Honorhall but, it never happens... Maven Black-Briar has never contacted the Dark Brotherhood... some annoying static aspects of the game I'd like to see better worked in a DLC.

Would be nice. I'd like to see Helgen rebuilt, too.

The fencing (for want of a better word) around the Gildergreen tree is partially destroyed, and Severio Pelagia and Heinskr's houses are destroyed. During the siege, several buildings and market stalls are on fire, but there doesn't seem to be any permanent damage.
I'm not sure if there's any permanent damage on the outside of Whiterun ...

there might be some damage to solitude/wildhelm depending on what side you take, but i didnt check

i dont think the area outside the walls of whiterun is damaged

nothing at other cities afaik, coz nothing happens there, the guards/jarl just get replaced and thats it

I don′t think this will happen. See Kvatch in Oblivion. It is really a pity Bethesda didn′t think about that even there were already complaints about Kvatch.

Well Kvatch was far worse then Helgen or the other cities since that place kept on burning for months, at least they didn't do that... but yeah.. Bethesda still has some things to do in my opinion.