Core Gameplay changes in Constellations.
By default, your character is the Dragonborn. This setting can be changed in the Skyrim Unbound MCM. Non-Dragonborn characters may not learn or use Shouts and don't have access to some questlines.
The Skyrim Main Quest is only available to Dragonborn characters, and will begin the moment the player kills their first dragon. Bleak Falls Barrow is completely unrelated to the main questline.
Dragons will spawn based on many factors, such as player level and overall time spent in the game.
Enemies do not scale to your level.
This can make some areas and enemies too difficult at low levels.
Don't be afraid to run away and come back stronger.
Fast Travel is disabled by default in Constellations.
However, Carriages and Boats have an extended list of destinations, and are added to several locations in Skyrim.
You can enable Fast Travel in:
Requiem MCM → Atmosphere → Fast Travel Allowed
Once a Birthsign is selected, you may only select Birthsigns within that Birthsign's group (Warrior, Mage, Thief, or Serpent).
See cnst-birthsigns for more info.