The Dreadnaught has a lot of potential to it. It could basically be a Hive version of the Leviathan.
The Cosmodrome, because it's relatively small compared to other places, would likely need a drastic face-lift again much like what it received in Rise of Iron to make returning to it valuable. There also isn't really a lot of strong lore potential tied to the Cosmodrome. They had to invent a plot device in order to make returning there reasonable, and we all but neutralized that plot-point.
I'd rather see a new environment brought to light (An extreme cold arctic-esque environment) or a return to regions that either haven't already had a revisit or have good story potential.
Y'know, when you say it would be like a Hive version of the Leviathan, it makes me picture it returning not as a full patrol zone, but a location we visit for some Endgame activities.
And I'd be okay with that. A new raid there, Crucible maps, an endgame activity, etc. Not every location needs to be a full blown patrol zone.