In ArcGIS for Desktop you can do some great animations by joining an attribute table of temporal data to your feature class and making use of 'time aware' layers. This is great in instances where the spatial object isn't chaging (e.g. an administrative unit) and only the underlying attributes are - you only have to store the geometry once. In ArcGIS Online you can't do that. You need to flatten your data model and have multiple copies of the same polygon for each time step. Can AGOL be upgraded to support table joins and also time-aware layers that are making use of table joins?