You can stake the bigger branch , support it so it stais vertical and now you have a new leader. Plant will focus its energy towards it and by pruning you can force it later to produce side branches.
The other idea i would have is to graft a desired variety in the bare middle, if it takes it may become a new leader, right now the plant probably wants to have growth in the center but may be too weak to wake up the dormant buds that may be lurking there.