I think a new Panzer game would need to be a rails shooter - it's what the series always has been (apart from Saga). However, it could be tweaked.
What if a stage has multiple entrances and exits, all leading to different places? And the gameplay would be mostly on-rails shooter, but with more overt hints at an RPG?
Let's take this for an example. You're in a town (on-foot, as in Saga) and want to get to another town. You stock up on the usual RPG stuff (potions etc) and go to your dragon to get to this other town.
Between the two towns is a large canyon level - a veritable maze. Your dragon constantly flies forwards (as in Orta, you can change speed and perhaps type) and you choose different routes as you go.
Now, to make this more like an RPG, there are lots of routes which allow you to discover different places. It could be that one particular route leads into a cave network, or a caravan where you can set down and do more trade. You don't need to discover that caravan to complete the game, it's just an optional thing.
Your route is also mapped out, and you can even purchase a map from various towns. Use this map to plot your route in the future and put direction markers, telling you which route you need to take. So you can travel again from one town to another, but in the reverse direction, and see what score you get. You may have to play this area again to get to the tunnel network, but basically it always plays as an on-rails shooter (though perhaps you'd need the ability to stop, get your bearings and turn around).
As for "a new game won't sell", I'd say it could, but Sega would need to be cautious. The demand isn't there currently. That's not to say it couldn't be recreated, especially if they could do digital re-releases of all the games so far. Then, perhaps, a new game (even if digital only) could be a good idea.