Writeback for Power Pivot /SSAS Tabular models
Either through a PivotTable, Excel cube functions, or Power BI tools
If done right, writeback can add a lot of value to tabular models esp. for planning scenarios.
Javier Recasens commented
You can do Tabular Write-back with this Excel add-in:
with splashing functionality for i.e. budgeting planning
that will be a great addition to the SSAS tabular model capabilities
Please do this! Tabular is awesome... but is it really missing writeback??!