And risk to have another civil war?
How you convince the provinces that it's a good thing?
Because after all to finish the Civil War an equilibrium was needed and had to be kept.
For a start, the area of the Pampas south of the Salado River and north of the Rio Negro river didn't became properly civilized before 1900, before this it was too scarcely populated and was no way a place where to have a goverment.
Then you have the problem of making Buenos Aires too powerful. I mean, Buenos Aires province was powerful even without the city. With the city it was almost impossible to defeat it. The provinces won't allow for Buenos Aires and the surrounding areas to be in the same jurisdiction.
My bet for a better capital? Santa Fe. It is more or less in the middle of the country, has access to the Juramento river, is at a considerable distance from Buenos Aires and is not the economical center of the province(Rosario). Also, by being next to Parana river, the authorities can send orders quickly to Buenos Aires, which a capital in another part of the country would be unable to do.
But something would have to be done with Buenos Aires. And it's going to be difficult. The only possible solution would be to cut the province by the Salado River. Which Buenos Aires won't like and will want many concessions in exchange. This way it could be done, but it would be difficult, and much would depend in how much control Buenos Aires has over the government anyway. Remember that Buenos Aires elite will still have the money and the interests. All they need is to bride the goverment.