Using "I" as the starting sound:
* Iridescent Islands
* Icy Inferno
* Intricate Ivy
* Illustrious Imagination
* Impassioned Inspiration
Using "I" as the repeating sound:
* Ice-cold Icy
* Ironing Iron
* Indigo Ink
* Inquisitive Investigators
You can choose the alliteration that best fits the context you're using it in!