It would be a Pre-LOTR story; will have Haldir/Thranduil, and M-Preg (Perhaps not explicitly, but at least mentioned or implied).
Legolas is relatively young by elven standards, probably a few good years away from majority. But for training as a warrior and or prince and or ambassador (Up to the writer to decide) he is sent to Lothlorien. While there, he forms a friendship with Haldir, and comes to look upon the older elf as a mentor of sorts.
It would be interesting to have the story written from Haldir's Point of view, but not nessecary. As it will be revealed that Legolas is the lovechild produced from Haldir and Thranduils affair. When first meeting Legolas, Haldir will know it is Thranduil's child from resemblance, but will not know Legolas is his child as well until he finally breaks the long silence between him and his old love. After learning this, he will keep the knowledge to himself for a while as he comes to terms that the young elf is his son (Shocking experience, you know.
The revelation to Legolas about his parentage will upset him, and angst ensues, as well as a rekindling of Thranduil and Haldir's love as they are both required in one place to fight for thier son's understanding.
Serious fic, good Thranduil, Happy Ending. Other twists and turns optional to the author, this is just a platform to build from.
> Haldir/Thranduil Characters: Haldir of Lothlórien