At risk of drifting too far into architectural theory, there are two basic approaches to themed structures that can be used to describe the differing approaches we're finding in Future World: the "duck building" and the "decorated shed".
Decorated shed: essentially just a plain structure with signage or a façade on one side that announces what's inside. Once you get past the initial front view, there's really not much to these buildings; the sides and rear are often shielded from public view. The interiors of the building are divided in any number of ways, typically with no relation to the building's overall shape. Think of your standard big-box stores, the false fronts on old west buildings, WDW's Value hotels, or Disney attractions like the Haunted Mansion or Small World.
Duck building: named after a roadside stand shaped like a duck that sold ducks and duck eggs, it's a type of building whose purpose is reflected in the form of the building. These buildings can be seen from more-or-less any angle and still reflect that purpose; they were designed to be seen and interacted with from all angles and are completely themed buildings. The spaces within the buildings often reflect the outer shape of the building, with domes, atriums, and other features that reinforce the overall form. Think of old-timey roadside attractions, traditional churches and cathedrals, the Contemporary A-frame, or Disney attractions like Space Mountain or Cinderella Castle
Disney attractions tend to follow the "decorated shed" approach to design: their park-facing sides are intricately detailed, but quickly fade to plain warehouses once out of view. This approach can be found in every park, including most of the attraction spaces in World Showcase; "decorated sheds" aren't inherently bad, but need to be used mindfully to create a themed environment and surrounded by smaller "duck buildings" to fill out the area.
The original Future World pavilions took the "duck building" approach (eschewing the smaller buildings in the process), with a holistic design where the shape of the building informs the form of the space inside it (or conversely, the need for certain spaces within dictate the overall form of the building), and reflects its intended purpose to passersby; this was a pretty dramatic shift from Disney's previous efforts, and makes FW somewhat unique among theme parks. Mission:Space and the new GOTG add-on building will follow Disney's typical "decorated shed" approach, stepping away from FW's original design ethos, and heading in the direction of making it just another themed area that could be found in any park