Although long, multi-syllabic names are becoming more popular with new parents, I don’t believe they will become popular in character naming outside of Fantasy.
Multi-syllabic names are difficult to say, and will stumble readers and slow the flow of the story. Granted, the writer could give characters with these names a nickname, but that could just confuse the situation, especially if the nickname isn’t the most obvious choice for the full name and both are used throughout the story.
Take, for instance, the name Penelope. I know two baby girls with this name. One goes by the nickname Nelly. While Nelly may be an often used nickname for Penelope, it doesn’t read the same at all. Not only do the names have a different feel, but they look completely different. So a story using both interchangeably for the same character may be confusing to readers.
While I think Marisilia is a perfect name for the sea witch character who was born in Renaissance Italy, I do think some readers will nickname her something like Mari, so if the writer also has her called Sila this is only going to compound the issue.
The longer and more complicated the name, the more likely the reader is to call the character by a shorter name or even a gibberish name. If possible, make it easier on your readers by using one name primarily for your character, or giving your character the most obvious nickname (like Penny for Penelope).
Keep it simple, to keep them reading.