Jatt has recently reveiled a lot of information about pantheons and the system they will run on in game when the mechanic is finally added.

"there will be no member limit to pantheons, it just leads to crap like "DerpClan1" "DerpClan2" and so on.. so no limit to member numbers.. pantheons can be set up in a few different ways by the pantheon founder.. such as fully democratic where a majority is required for any pantheon decissions.. or a free-trade agreement where each individual empire still has all-but-full autonomy, and can declare wars on their own without dragging the pantheon in, but if a member is attacked the entire pantheon is dragged in as defenders... to dictatorial/elite where one or a few empires set the rules for the entire pantheon
regardless of pantheon type, they can also set taxes for members to fund various mega-scale projects and pantheon supported fleets
war or no war does not change visibility of a system at all, you see what you see
and yes one of the options in a pantheon will be to allow full collective rule of all member empires, effectively, the pantheon becomes "one huge empire" with each member player being able to control anything they want
there are upsides and downsides to each pantheon system, and treason is expected"