also
I don't know what the fuck that is, but I want to play as it.
Not a fan of the color choices on the fasces though.
That seems highly unlikely. For historical games you can add more history/map areas in DLC, but the only thing they can really improve in a 4x is the mechanics themselves. Whilst I realise there are costs to balance so they can't be too ambitious, I do feel that they're basically setting the game up for DLC which is a business model I really dislike.
There will definitely be major mechanics that end up being DLC, but they can add the equivalent of history to the game no problem. Look at the kinds of small things that they added to CK2 and EU4. CK2 got new playable religions and government types. Stellaris doesn't have religions, but it conveniently does have government types, and I'd be willing to bet that civilization types (ie synthetic and hive mind) stuff will get involved in expansions as well. Now consider what EU4 has gotten. Yes, its expansions tend to be lackluster aside from the mechanical aspect, but most of them deal with a specific aspect of life, or a region, or a cultural/religious group. There's no reason they can't do any of that for Stellaris, just swapping out "region" with general cultural blocs and types of empire as needed. Also consider that the game is randomly mapped, and look at what Stardock has done with their fantasy games. It's relatively easy for Paradox to make DLC which incorporates new elements for the map to draw from, as well as things like event packs.