Honestly, and dragging this off-topic slightly, I expect SHO to go down the route that Star Trek Online has done. You start with a Type II, the whole map is open to you, but some areas are safer than others. You probably start in the Nordsee, where if you run into a ship/warship it will be capable of being defeated by a Type II, but if you take your Type II to, say, the Western Approaches then you run the risk of running into something a bit more deadly, a Black Swan perhaps, or aircraft which a Type VII might be able to take on just fine but a Type II can't.
In terms of advancing the years, perhaps they will do it in a three RL month per game year, or something like that, scaling it so as more people level up then more advanced U-boat variations are released, or more technological toys are released to upgrade your boats with (Snorkel, sonar, torpedoes, etc).
Paying for stuff might get you there quicker and avoid the grinding, and you might be able to get some unique boats with extra tips (perhaps a special 'Kretschmer' boat or 'U-96' boat) but it will still be accessible to those who don't pay.
