A displayed remaining total ad duration, in combination or in place of the "(2 of 6 ads)" at the top of the screen.
At this moment when ads are played you get a timer for the single ad you see. Next to the (<ads seen> of <total ads>)
A good addition to this would be a "total time remaining" counter for all the remaining ads combined.
Right now it can be very off putting having a 30 second ad, and seeing that you still have 6 ads to go. You have no idea how long you're gonna be stuck in ads for. Does that mean we're gonna be stuck in ads for 3 more minutes?
Even if the next 5 ads are all, short 5 second ads. Without seeing what the duration of that is gonna be we'll expect the worst.
What would really improve the experience of ads, would be knowing how long you're gonna be viewing these ads. Knowing that you're gonna see ads for 120 seconds, feels better than seeing that you're on ad 1 of 8.