Rebuilding Helgen would be awesome; don't know if you could be Jarl as Helgen is already in one the Holds and isn't big enough to be a city with its own Hold, but you could certainly be a Baron of some sort.

Yeah, Helgen is already in a hold so becoming a Jarl after rebuilding is, in my view, not lokely to become possible.

However, it could be feasible in a "build your own stronghold" scenario.

I'd like to have the type of "build your own stronghold" logic discussed in this and the previous thread I included above and, in relation to the "my actions have impact on the world" want that at least some players have, the ability to rebuild Helgen and the ability to invest in businesses in smaller towns and have the game react dynamically to that, i.e., new businesses attract new NPC residents who build homes and expand the town would definitely provide a "my actions have an impact on the world" feeling.

I'd have to go to Blackreach and name my stronghold "Blackrun"

Maintaining a stronghold in Blackreach would definitely be an interesting challenge with Falmer skulking about.

I have a feeling they might not be welcoming to a new non-falmer resident on their block.