I think there are a couple of underlying assumptions in your post that I don't necessarily agree with. Also, the following assumes we're talking about 2010, which it seems like we are.
If you're going to make a new ship, it has to occupy a new role than what is currently available. Otherwise, why bother? It's less about "covering all the bases" and more about addressing deficiencies .
Keep in mind, the orks can already get a 6 hit light cruiser. They just never do, because it isn't a very interesting ship. It's just a Killa Kroozer on a diet. So the size class is already there. Just the reason to take one isn't.
As for the marines, they can take a ship intermediate between a Battle barge and a Strike Cruiser. The Venerable Battle Barge can be any imperial or chaos battleship, grand cruiser, battlecruiser or heavy cruiser. Sure, it may only be one ship, but the option is there.
Interestingly, the marines are provided with a cheaper Strike Cruiser (the Vanguard Cruiser) in the Badab War Forge World book.