I am surprised to see this wasn't requested before and would like to be brief initially.
One feature I really need as a part of the financial software package is Asset Allocation. There is a lot written on the concept and basically means we assign each investment (stock/etf or other) to an asset class. Then we assign a percentage this asset class should be allocated out of our overall portfolio.
Updating prices allows us to see how asset class allocations move and how much are they offset from out desired allocation.

I have the idea implemented in Money Manager Ex for Android.
Technically, this could be implemented as another field on the asset/investment account and a report which displays the dis/allocation (the pre-set allocation and the current allocation, with the differences between the two).

I am happy to contribute implementation in the mobile version and am hoping someone would be interested in implementing this in the desktop version of GnuCash.
Cheers