The two most requested features I've heard for MtgHelper are:
1) An in-app card database / deck builder
2) Life total history
While I feel that a card database or deck building app is much more suited for a stand-alone app than something to tack onto MtgHelper, I have considered life total history.
The reason for this is that having a history of life total changes can be helpful at tournaments. If there is a dispute over the life totals, it allows you to back things up and recall how the current game state was reached. This is the reason many serious players use pen and paper instead of spindown life counters or apps.
The issue with such a feature in MtgHelper is that, like many apps, life is ticked down one at a time. It would be annoying to see a long string of +1's or -1's on your life history, so there has to be a way of eliminating this and consolidating them into a single +5, or -3, etc.
This could be implemented using a timer (slightly worse on your battery, I guess), or I could force the app to only allow long-presses to change life totals while in history mode. This would of course be an opt-in functionality and would not be the default.
I am very open to suggestions and/or feedback on this, so please let me know your opinion if you feel strongly about this!