Together with my team we have been using asana and we love how it helps to organize our work.
However, at the end of the day, everyone is different and have some specific preferences of working. For this reason, I believe there should be personal custom fields. Meaning, custom fields which you add (probably in My Tasks) and are visible only to you, and have the same functionality as in projects: eg. sorting tasks, view and edit the custom field next to the task name, etc.
The example that started the need for me is daily planning. Having a custom time estimate for task completion helps to know in advance how much you can accomplish. I tried to achieve the same by creating a private project that quickly I discarded because: 1) Custom fields do not appear in My Tasks; 2) My Tasks can not be sorted according to custom fields and; 3) It is hard to maintain My Tasks and a separate project which implicitly are designed for the same thing.
I am sure the community have tons of other use cases in which custom fields are needed for the individual, but not for the project/team.