The interview is important in knowing player expectations and ideas. A cordial and casual get together with drinks and tossing around cool ideas helps the GM have an intuition on the boundaries and concepts the players would like to see. As a reciprocating process the GM gets to also see if his own ideas fly with the players'. Its better to get assumptions out of the way, before the work begins in setting creation.
The session of brainstorming will slowly move into specifics. The GM would first look at macro, then towards details. Details like, how powerful or cinematic will the players be? Realism and Science are tools that aid players in such settings because realism is easier to argue because it is more objective than notions of cinematics and taste.
How the players visualize or expect themselves to advance matters alot. Players who like to keep it gritty see advanced characters as just wiser but at the cost of mistakes that leave permanent marks vs players who see themselves or characters advance to become the new powers that be.
Note that, Its always easier to work with players who enjoy the Journey more than the Goal. Accumulation of Imaginary power and one-ups-manship is common negative trait in RPG gaming, and spotting such expectations early matters a lot in matching expectations with players.
Calibration is an important aspect of A new venture. First sessions and opportunities for the Gm and the Players to see eye to eye should always be relaxed. Expecting the players to "Get the GM" immediately is asking for omniscience and the players to be mind readers. getting a feel of each other style and expectations is always an important. Any deviation of expectation, when the GM experiments has its credit and fault fall on the GM.
Sample Kitchen Sink Premise: kitchen Sink setting, is basically an alternate sci-fi where an event changes the progress of humans from space flight and colony to exploring dimensions within earth's time-space anomalies. Never having to develop spaceships, instead air ships, submarines, and tunneling transports that can carry massive amounts of capital from alien and conflicting parra worlds.
ecological spill over puts monsters into the earth. At realistic power level, humans are tech dependent. genetically altered humans have magic/psionics implants that is basically technological abilities in bioform. Of course these all require a powersource that the body can metabolize through direct blood infusion. All bodily function requries a metabolize energy source.
Brains with telepathic is actually a special biological radio that genetically adapted ability to translate and understand it. Needing a powersource to broadcast, special blood infusions are created to supply the function.
TK works the same way, so is Pyro kinetics etc. These all depend on element X which hare a valuable resource. Because tech is organic, it is self repairing and is designed to evolve to greater sophistication.
Humans are late adapters of this biotech, but still retain their mechanical technology. Elves have forgotten their other tech over centuries of dependence on their excelent innate biotech enhancement/aka psionics. Dwarves never liked the idea and have focused on genetic hardiness and retaining their mechanical and electronic technology. They come from high G "world" where they are specially adapted to.
Dragons exist because of parallel worlds that can support them. They also evolved intelligence and symbiotic technology. And throw in Hive mind Virus that needs Hosts as the undead. Other aliens can fill the fantasy stereo type. Parts of humanity Lost in future past version return: A Roman empire, a Persian Empire, a Age of Sail Empires, waring states.
Where does Low Tech function? Some areas strong radiation that interferes with non-bio chemical electric devices. Some places and areas require mechanical tech only. Some places cannot support Industrial tech because of Logistical problems. Without tech, the Biofuel to augment psionics/magic is very low and rarity of strategic resources like an Iron industry and inroads make the area very bare.
Time Space anomalies mar geographic features of the planet giving it the surface area of several worlds while maintaining near-earth like conditions.
Countries who used to occupy certain spaces have their time space messed up and have a lot of time advance in their area morphing them to something entirely alien yet familiar. In certain areas, 100 years pass in isolation, other 200-600 hundred, others just 20 years etc. In such times up becomes down, and whole cultures turn around or find a terrible fate or earn their destiny.
Begin with one area and a whole lot of ideas that make it past the cutting room floor. Then slowly build up with a solid set of premises. Pepper with preludes and small subtle foreshadowing of certain concepts.
When the setting is developed players can enter as Freetaders! Trading tech from X to Y, and dealing with a whole lot of barriers from monsters, politics and intrigue, and sci-fi fantasy aberrations.
All purpose environmental transport are in the form of Hellium Air ships that can stay safely out of reach of many land-terrors. "fighters" are ultra light armed fliers designed to take on sky predators.
Characters requiring MWs of metabolizable power to use their psionics is a easy way to balance "magic" and tech. This is because options and ability to do certain things scale at the same rate as technology. So a Radio and a Telepath are on the same par. except that a telepathy is more handy in tighter situations. Realistically speaking, biological functions that eat up physical space in the brain narrows intelligence and mental capability. All follows the economic rule of cost and trade-offs.
In evolution nothing is for free, this is also true in Physics in laws of conservation. All you can really do is make certain things more efficient, by rearranging things and getting rid of vestigial adaptations that are not anymore needed and which technology does a better job at.
Introducing Logistical Limitations to tech is important. Anyone who knows the wear and tear mechanical parts go through as they get more sophisticated can apply a simple bell curve of diminishing returns on high tech gear. the more high tech the more support. The GM fixes the equilibrium to his own more ideal tech level. You still have power armors, mechs, and sci-fi weapons but the economic cost is only affordable to few places and people.
Awesome armor Armor is Ablative, making resources a Juggling Act and part of the challenge. This makes players either work on balancing the budget or go for simpler technologies which is easier on the GM. The objective is making an easier to run kitchen sink setting.