From the old games,I think there could be a "best of both worlds" when it comes to to both Mass Effect's exploration mechanics, a fusion between the two: scanning a planet for a suitable ( read: quest-related ) landing site, where you drive around to accomplish the objective, or just drive around to admire the view.
However, if there were a situation where I had to pick either of the two, I'd settle for the old Mako driving- tighter controls, and believable physics would only make those segments better. ( As a side note: The Mako is not a tank- it's an APC- I believe that the Codex mentions that )
But, throwing it up there, seeing as the Normandy is equipped with a hostile insertion vessel maybe a boarding mechanic could be implemented, if you as a player have the appropriate psyche ( again, throwing it up there; ruthless and a long history of being a renegade ).
And, to make the package complete: some ship-to-ship combat. What we've seen before ( Freespace, Freelancer ) wouldn't cut it- Hey, the Normandy 2 is -huge-, don't expect it to be able to tail anything smaller than a cruiser.
It'd be interesting to see Shephard acting as the Captain of the ship, supervising the battle as the crew make it happen.
It might be too much, too different from what Mass Effect is; squad-based tactical shooting, but it'd sure sell me on the fact that yes, there are other ships out in space.