With more complex solutions using GIS are used with out of the box ESRI products, repeatedly we come across where the data structure is many table records for a feature. This of course is handled in ArcDesktop.
Implementing full Relationship in a web stack to mobile environment would open up more use of collector.
Table-to-Feature (1:M) and the ability to edit these tables. Recent release of 1:M Feature-to-table is great... this would be the finishing touch!