Currently there is no other format respect to xlsx which can mantain all the PowerPivot features of the workbook, could you please tell us your true requirement? Among the self-services BI tools, besides PowerPivot, Power View is also a good choose, which have more relationship with PowerPivot, which is an interactive data exploration, visualization, and presentation experience. It provides intuitive ad-hoc reporting for business users such as data analysts, business decision makers, and information workers. They can easily create and interact with views of data from tabular models based on PowerPivot workbooks published in a PowerPivot Gallery, or tabular models deployed to SQL Server 2012 Analysis Services (SSAS) Release Candidate 0 (RC 0) instances
For more information, please see: http://technet.microsoft.com/en-us/library/hh213579(v=sql.110).aspx
TechNet Community Support
Hi Challen, if you go at the site www.powerpivotpro.com you could see a PowerPivot hosting solution, without using SharePoint. It seems that the first step to implement a such manner to publish a PowerPivot workbook it is represented by an saving as operation for the workbook.
This hosting solution is related to first version of PowerPivot.