It is perhaps the least drawn-out part of the Silmarillion, there just isn't much plot to it. What I'm thinking is, if it were filmed as Part 4 of a Silmarillion series, would it have enough storyline to stand on its own?

As long as you put the results, it certainly is!
The Silmarillion is about the Silmarils. Just as Lord of the Rings is about the ring. You need the voyage of Earendil paired with the final overthrow of Morgoth to truly end that series. No matter how many films you make out of the rest.
Consider the big three... noone defeats the bad guy! Oh, they win battles, get back a single Silmaril, but still Morgoth is there. And he's not dying of old age either.
You have to have his voyage, the Valar returning, and the last two of Fenor's sons die. You need to end the Silmarillion with one in the sky, one in the sea, and one in the ground.
I guess you could skip the Valar creating Numenor during the epilouge though. But I would love the prospect of a feature film quality retelling of the Akellebeth.
If you didn't use Earendil's voyage, how would the story end?

Agreed The Voyage is essential.

I also would like to see "Akallabeth" on screen...hmm...methinks of another idea...

A quick reread of your question made me think. Even as only one scene in the film, how would they actually portray that voyage itself?
Would he wear the Silmaril on his brow and just sort of make it through the enchanted isles?

Perhaps they can show him enter a horrific storm, only to be beguiled and end up heading back east again. Then after (Damn, I forgot his wife's name!) arrives on his ship he tries again.

As to how the filmmaker would like to show her take the hue of a bird and fly to his ship? I have no clue. But that is for another discussion.

This time, when he enters the same horrific storm the light of the Silmaril shines the brightest and allows him to make his way past the Enchanted Isles and out of the storm. And then on to the shores of Aman.

But what can they do to the storm to make it worthy of his journey? What could they do that would make it bigger than just a repeat of every other film storm. How do you represent a end all, kill all storm?

Elwing is the name I tink you want Show

To represent the kind of storm that the story needs, the filmmakers need to treat the sea as an actual character. They need to picture it as a character that Earendil is actually fighting against to get the the Valar. I'm not talking about a sea-god like in Prince Caspian (he's not fighting Ulmo). I don't know how to put it into words. But the mood of the scene can't be a normal storm. Normal storms are forces of nature that we have no power over. We must just wait them out. This storm must be something that Earendil, with the help of the Silmaril, needs to subdue, not just survive. I hope that makes sense.
In the grand scheme of things, it would be very difficult to cram this into the Big Three. In order to make things complete, they need a film before the Big Three, and then a film (including the voyage) after the Big Three, making a total of 5 films.

I don't believe the travel of Earendil woul make such a good movie by itself. To do so, you would have to tell a lot more about his journey past the enchanted isles, and that would be too much like the narnian Voyage of the Dawn Treader, or the travels of Odysseus or Gulliver.

A more promising story would start of showing the reason for the voyage: The elves and edain losing, on retreat. Then the voyage, with a mighty storm, Elwing changing into a great bird and flying out to meet Earendil, a short part of passing through the enchanted isles (we don't know much of what goes on there anyway), and arrival at the shores of Valinor. After an evocative passage about the empty city of Tún, the armies (and gods) of Valinor go to battle. A climactic battle follows, that dwarfes (sorry Gimli) the battle of Minas Tirith, and the overthrow of Morgoth. After that Earendil sets sails across the sky, and as the light fades Elwing changes form and flies after him...

In this movie the voyage still plays a key role, but only as the catalyst for the intervention from Valinor. And if anyone wants to make that movie, I'm game for the manuscript...