I would like to see some serious changes to Data Sense. As it stands in 8.1, it is useless if you have a shared data plan with multiple phones on the plan.
It should keep up with all the same usage data, but you should be able to configure an option as "shared data plan", and enter the Microsoft Accounts of all phones on the same shared data plan. Each user would then confirm they are part of that plan. Once all users are confirmed, you would be able to track each individuals data usage with overall summaries for all users. This way you can know at a glance how much shared data remains for the rest of the billing cycle. Data would have to be stored in a database online in one of the users OneDrive, with each person designated as part of that shared plan having full read/write access to the database file.
Other options for settings are also required. Some people have a monthly recurring data plan, and some people are every 30 days, others are every 7 days. An option for daily, monthly, and custom number of days should be added. Some people purchase an amount of data, and that data is good until used. This should also be handled somehow.