I agree with some of what Aanchir said - it would only have a decent chance of coming back with a fresh start... maybe all the way back to the unique and awesome combination of islander and robot that made it so amazing in the first place. I think it could be re-released as a major movie, much like with Transformers - get Hollywood directors and a ton of brilliant effects and fight scenes together to remind the current adult population of something that was cool when they were kids.
As a consequence, there would have to be some adaptations to fit the screen, like making the Toa and Matoran into creatures of flesh with robotic skeletons and a few important details, like one or two exposed pistons and glowing heartlights. Masks could change to face-painting for the Matoran, maybe, with full details for the Toa. People like to see people in movies, after all - even superheroes spend at least half of their screen time out of costume. Plus, to be honest, they would probably want to cast some attractive stars and have characters who can take off their masks to show some emotion.
That said, these changes would affect the toy re-release as much as they would the story. There could be a medium between the smooth, sleek parts of HF and the industrial, get-r-done aesthetic of the original Toa, mixed with the details of the various elements. How exactly they would do this is beyond me - the Lego system will change considerably in the time that it takes for this plan to line up, and they might have parts and technology that would fit the demand perfectly.
Finally, to recap what I said at the beginning, the story should go back to year one: the Matoran are suffering from rahi attacks organized by Makuta. This is how I envision the movie working:
- In the beginning, a narrator gives some history in the style of Turaga Vakama in the story. Makuta is evil, etc.
- Someone summons the Toa at extreme personal risk, probably racing rahi to the Kini Nui at night. End scene with a flash of light.
- Morning on a beach. Canister washes up, its contents explode out and assemble themselves into a Toa (which one, I don't know), growing organic parts over the robotic skeleton as if fuses together. The Toa arms and equips, then heads inland.
- Gradually the Toa make contact with each other, the Matoran and the rahi. Some Matoran stars are introduced as well, possibly the Jaller-Halhi love interest.
- The plot develops with mask-hunts, rahi fights, and drama between the Toa until they are ready to enter Makuta's lair.
- As the climax approaches, plot twists emerge to throw experienced fans for a loop (Nidhiki is one of Makuta's servants, Metru Nui is mentioned in different context, etc.)
-The Toa fight Makuta in an epic battle, the plot twists again, drama/love interests are resolved or cooled and the movie ends on a high note.
-The credits roll accompanied by film score, a bonus scene hinting at something in the next movie is snuck in, and the movie ends.