check your MIA-Carribean schedules. Most of the islands that have 2x daily flights have an evening departure that is generally after 5 and most often after 7 (see e.g. BGI, SJO, AUA, GCM, STT, etc.). That leaves 3-6 hour layovers from CHS. It misses all the morning late-morning Carri bean/Latin American departures which generally includes most destinations which get 1x daily. Its also far too early for South America.

JFK-CHS will likely never happen with the current management which is super-conservative about flushing out the network and instead is more interested in adding random spokes. But considering the number of LGA-CHS flights, they could likely make it work with a combination of NYC, Europe, BOS and even west coast traffic (e.g. CHS-JFK-SEA).