Is there any possibility to collect height information to feature service (Either as attribute data or as z coordinate) now or in future? Nowadays you can use external GPS (DGPS/RTK) together with Collector and it would be useful to collect accurate height data also.
You can put your ideas with your user case in http://ideas.arcgis.com/ and also contact Esri Support to create an enhancement request.
Have you tested Collector with a 3D-enabled feature class to find out if it will capture an elevation value?
There is the sxBlue III GNSS which I've not used but is supposed to offer cm accuracy and work with the operating systems that Collector lives on.