What's a Good Thesis for Comparing LOTF and Coral Island based on characterization? The Thesis must have the following: -Perspective -Beyond the Obvious -Why? -Analysis
If you are serious about making the comparison, you really need to read "The Coral Island." You can get decent, slightly abridged editions; the original is something of a tome. Also, I recommend the essay, "The Coral Island Revisited" by Carl Niemeyer, published in College English, 22 (January, 1961), and in the Casebook Edition of "Lord of the Flies" (Perigree Books: New York, 1988)
As you will see, "The Coral Island" is very pro-British, very Christian, and very moral. Golding has admitted that LOTF is something of a reply to this Victorian view of the world, in which all adversities can be overcome with hard work and a positive attitude. But in forming your thesis you need to address the complexities of Golding's novel, which are legion compared with Ballentyne's.
I don't want to write it for you, but a good direction would be to use the multiple levels of parallelism between the novels. Golding's characters are excellent antagonists to Ballentyne's. At every conceivable point that the Coral Island kids overcame obstacles and maintained discipline and civility, the LOTF kids failed and fell apart. This started with the naming of the characters (Golding uses the same names for this three main characters but they are almost foils for each other) and continues until the end where Golding's characters are killing each other.