Hiccup realizes his calling of becoming a dragon trapper, since he would never be able to kill one.
He convinces his dad to let him leave to join the Trappers, (maybe he has to pass a test of capturing at least one dragon). Not Night Fury!
Maybe some other teens go with him ? Astrid a chance to take the fight to the beasts she couldn't miss that.
Eventually teams up with Eret, son of Eret and invents the launchers seen in the movie.
Astrid x Hiccup - Astrid x Eret - no romance at all any could work
Meets Drago, his traps and machines are more advanced that Hiccup's. (He doesn't become some special apprentice Dragon Master he is still an underling like everyone else).
Hiccup and Eret's fort is eventually attacked by Valka, Hiccup captures Valka when captured she hears someone call him Hiccup. Realizing who he is she escapes the nest or trap and summons the Bewilderbeast.
She and Cloudjumper fly off with Hiccup and take him to her nest.
She tells him who she is and tries to convince Hiccup about dragons, this might work, and this might not.
(This is a chance to have hiccup ask the questions we all want to know why she did not come back, why she did not do anything about the raids or the Queen.)
Could have the "Let me show you" moment from the first movie with Valka and Hiccup
That's all i got. If some one likes this idea then feel free to take it and expand, change whatever you like.
Can't wait to read it.