Legacy can be a potent force.
You envision a situation where your MC, overcome by the difficulty and dangers of his quest/journey has perished with his task half done.
The task is vital, so someone finds himself with greatness thrust upon him. He or she might not be too pleased to be the one who must continue this journey and finish the task.
I suggest that you have a couple of secondary protagonists who, when your MC dies, can soldier on. They can fail and struggle and fear they will fail utterly - a better and stronger hero perished, so this is no easy feat.
Together, they manage and their flaws and strengths complement each other. One of these could be someone the MC thought little of so he has even more to prove.
Bella is right - the other characters must be interesting enough to carry the story. If you have only one character who is fully realized and the rests are filler, let your hero finish the quest.
If you have multiple interesting characters, kill the hero, but make it meaningful. His death can be a catalyst for others and inspire still others.
Years ago, a young cancer survivor who lost his leg to cancer decided to raise money for the cause of research by running across Canada. It was an inspirational story that was covered in the nightly news - his progress carefully measured. Donations began to pour in and people would meet him on the road and wish him well, cheering him on. He went far, but did not finish. His cancer was not completely in remission and it returned. He died still raising funds for others.
Two others chose to follow in his footsteps. One used his wheelchair and succeeded in his quest, the other started from the other coast and did what the first had hoped to do. Steve Fonyo people do not hear about, nor do they talk about Rick Hansen - but every year in communities all over, there are Terry Fox runs.
He raised more because his brave effort failed, yet was so inspiring. Everyone who takes part in a Terry Fox run is helping raise funds and doing it in his name, in his memory. Had he succeeded in crossing the country, he would have raised a lot of money, earned accolades and been highly esteemed, but his death made people wish he had succeeded and choose to try and help.
Make sure you have characters who can pick up the torch and not be crushed as they carry it onwards.
If I were doing it, I might have a scene where a few of the others gather, mourning the death of MC. A few ‘without him, we are lost’ and then a realization that they learned something from MC. Without intending to, he prepared for this, teaching skills to some that would be needed.
A quiet realization that all are expendable and yet all are capable and valuable can come to one of them, who might explain that together, weak as they are, they will be stronger than even the MC since he was such a hero, always charging on ahead and doing the noble thing. Together, they are smarter and stronger and can succeed - if only they stay together.
