Some of the things to look at would be to examine your data types and to consolidate it based on the different types of data stored on your file-sharing environments. The other thing to look at would be how this data is accessed and what are some of the data access profiles.
For example, is this accessed on a daily basis, accessed once in a while or accessed in an ad hoc manner? So classifying data in a loose sense would allow you to consolidate it based on its profile.
The other thing to do is to take a stock of how this data is accessed. Is it accessed on a single network in a corporate office or is it accessed by remote connections? If they're remote connections, look at how having a file-sharing environment in a single home office can benefit remote offices.
Lastly, look at how much you are willing to invest in a single NAS solution that is a one-size-fits-all approach. It might just turn out that consolidation is an expensive option if you're going to invest all of it in one shot.
You may look at a tiered solution or a pay-as-you-go solution, which will allow you to start small, and as you start migrating stuff to the new environment you can grow. That way you're not making a huge initial investment and you have a chance to try it before you invest.