In some Skyrim videos I saw Ysolda taking care of the Inn in Whiterun. I do not know how this is, but what I do know is that Ysolda said she would become new owner of the inn once she earns money form trading with Khajiits. I gave her a mammoth tusk, so I don't know it that's going to make her merchant, but it there any way to make Ysolda new Innkeeper of the Bannered Mare?
Ysolda is scripted to take over the Bannered Mare upon the death of Hulda.
To make Ysolda the innkeeper of the Bannered Mare, you'll have to kill Hulda, or get her to die in some manner.
However, Ysolda is also scripted to take over some other shops upon the death of their owner. She will not appear in all the shops at the same time. I am not sure exactly what would happen.