Stellaris' game director Henrik “Doomdark” Fåhreus has started a series of developer diaries for Stellaris on Paradox' forums. In the first entry, "The Vision", he reveals' the studio's grand plan to "eventually cover the entire "human timeline" with our games... including the future."
Exploration, the "most neglected" X, will be important. Stellaris' randomisation features are designed to keep this phase as replayable as possible, to evoke a sense that each galaxy is "ancient and full of wonders".
Factions and important individuals like planetary governors and scientists will possess a wide range of traits. Technology is also randomised to a degree, distributed according to chance and the traits of your scientists. Traits remix the wants, needs and capabilities of each race, generating a varied galaxy and lots of potential for conflict. In theory, at least. Similar systems worked very well in Fåhreus last game, Crusader Kings 2.
"The galaxy should always be unknown and surprising. That is why there are no "major races" in the game, and such a great variety of discoveries you can make. In the same vein, there is no fixed technology tree."
Fåhreus hopes that the focus on expanding and scouting new races in the early game create a smoother experience for players put off by Paradox' dense historical strategies. He also points out that this is the first Paradox game to feature symmetrical starts for factions. When a game begins, every race is trying to expand and survive, just like you. That means powerful races you meet will have thrived int he same conditions as you, in a crucible of competing AIs. It's an exciting thought.