I think it'd be pretty cool if you could be the Jarl or King of your own Hold or island in one of the DLCs.

That would indeed be cool, fun idea we can only hope Beth adds it.

Kings and Jarls just sit around on their asses all day, what's the fun in that?

This.

Let's see building a hold, recruiting merchants etc for said hold guards assigning guards sending them out on quests defending your town.

And that i just thought of in a minute or so so yes many ideas can be done with such things, in SL when you became the daedric prince same concept and that was not boring.

It takes years to make a village and longer for a big city

Skyrim is suddenly following real life rules now? well i hope if i am ever to be beheaded a dragon swoops in and stops it.

It's a game a town can be built fairly quick, the hearthfire manor such a thing in real life could take months in game it took 15 20 minutes