Update: the next update will have a vnav feature like this. On Virtual Radar you can see your planned vertical levels overlaid on top of what you're actually doing, and it's easy to see whether you're above or below where you planned to be at any stage. You also receive alerts during flight when it's time to start a climb or descent. A new beta will be posted shortly.

One feature which is quite useful in Garmin's GNSxxx are the VNAV indications. Either having an alert ("start descending 500fpm to reach 3000feet before next waypoint", with "500ft" a customisable setting and the waypoint and altitude coming from the plan) or an onscreen field giving the fpm required to get to the planned altitude at the next waypoint. The latter may add confusion to the MD screen but would be fine on the iPad version