Well, none of the rules stipulate that you should keep the suggestions to something even vaguely plausible (i.e.: Something that there is any chance in hell that would actually be made), so?
Note: don't intend to criticize the original games or any of the work done for it. Just briefly indulging in wild fanboyism.
Story A long time later, in the same galaxy... Starkiller awakens out of a vat. It may be a cloning tube, or simply a bacta tank - it is not made clear. There are signs of an accident nearby, whoever awakened him is dead. He eventually finds out he is in Wayland, a lush world, done sandbox-ish, Arkham City-style. There are several local conflicts to get involved in (read: Side-content), but the core content concerns the Disciples of Ragnos attempting to take mysterious force powers from the world. Starkiller may ally or oppose Corran Horn, who comes into the narrative around the end of the first act. The Disciples eventually make a new clone of Joruus C'Baoth, who proceeds to do what he does best: Enslave everyone.
Yes, this takes place around the same time as Jedi Knight: Jedi Academy. The Force Unleashed was all about bridging different chunks of Star Wars continuity (Originally, Episode 3 to 4), so it stands to reason it could be used to bridge episode 6 to New Republic EU stuff. Using stuff built around the Jedi Knight game series and the Thrawn book series makes sense to me: People would be getting introduced to some of the best content there is in SW EU.
Note: It is left vague, more of a conflict outline. A lot of work would have to be done towards finding motivations for all the people involved, making conflict personal, etc.
Gameplay
The core systems and concepts from the original game are quite workable. If anything, I'd suggest:
- Relying less on QTE (Even if they look pretty);
- Visually toning down Force Powers (so enemies can realistically be seen to survive it).
- Make force-powers more specific and tool-like: They are different tools for different situations, not different flavors of death.
One system that should be considered is a means to use the Force to sense enemies. This would be useful in finding hidden locations, detecting stealthed enemies, finding weak points in large enemies or vehicles, etc. It could end up being as simple as Batman's detective mode. Prototype and test until something enjoyable is found, or dump the idea.
A stealth system would also be interesting - quite possibly using force mind-tricks and force stealth as concepts, so that it is more about using resources than crawling from shadow to shadow. Could lead to fun "cat and mouse" games against stealth-using enemies.
Of course, systems to support the shift from Stage-based linearity to sandbox would be necessary, but I suppose that is implied in the premise?
Enemies
- Stormtroopers: Hapless victims, same as always. Fun to kill.
- Elite Stormtroopers: The ones chosen and cloned by Thrawn. Tough, smart, enemies who used advanced tactics and weapons against the player. Yes, there were some hidden stashes of them, the Disciples dug them up.
- Noghri: Make heavy use of the "sensing & stealth" game systems mentioned above. If they get the jump on you, it is pretty much game over in most difficulty settings.
- Bounty-Hunters: Come in several tastes, and with two origin stories: Some working for themselves, and some hired by the Disciples. They are all quite adept at fighting force-users.
- Native Force Adepts: Apparently all that crazy force-use in the planet results in a reasonable amount of these. They are more "Shamanic" in style.
- Vehicles: Combat shuttles, crawlers, walkers. Add a game element where force senses can be used to find weaknesses in their structures and destroy them with precision, rather than raw power. Think Force Shatterpoints.
- Joruus C'Baoth: Apparently, the data used to clone him was never wiped. Final boss for lightsiders. Overall powerful, and keeps the fight interesting by messing with Starkiller's head (with the force, of course).
- Corran Horn: Final boss for darksiders. Specially tricky fight involving use of TK and the environment, as attacking him directly with energy attacks empowers him to crush you like a bug.
There is a pattern here. Notice how almost all the enemies are high-power stuff that has been shown in canon to be dangerous to Jedi? Yeah. That is deliberate. Make fights mean more, demand more (meaning using more diverse tactics and gameplay elements) and make it more credible when enemies become a threat to Starkiller.
Sequels, expansions
At least one of the endings results in Starkiller being stranded or choosing to remain in Wayland. That ending becomes canonical. Expansion or sequel takes place a further 15 years down the line, when the Yuuzhan Vong come a-knocking.