A prologue. A few hours of gameplay, enough to get a feel for the game and enough to get a hook into players, released for free a month or two in advance of the game itself. Maybe not concerning the protagonist, but rather some event that sets the main game in motion.
If P:E is intended to be anywhere near them, assurances like: "At this level, the game will be at least as long as PS:T" or "will have at least as much content as BG2"
Package "additional companion/race/quests/enemies/art assets" as "added exotic local". Going from Athkatla to a pirate island to the Sahaugin city to the Underdark - and having each of these act as a quest hub - was one of the coups of BG2.
Free DLC. "At this level, we'll produce /x/ free DLC packs with as much content as F:NV's packs" or "At this level, we will continue developing content for P:E for /x/ months after its release". Allows you to extend the development period without extending the development period.
Expansion packs. "At this level we'll produce a Tales of the Sword Coast/Throne of Bhaal/etc old-timey style expansion".
In-game community support. Ways to rank, view, download and install fan-made content from inside the game. Also, detailed statistics tracking and comparison: "You have killed more mermen than 97% of players", "You have reloaded fewer times than 6% of players", etc
Release of game assets under some copyleft license.
A small yet fully functional city simulatosaofffffffff