Looking to create a dropdown field to select an icon I use the same icons across most of my projects, and typically just list them in Dropdown fields. However, this quickly becomes difficult to manage, so I'm looking to create a simple plugin that I can easily reuse.
I've started with pluginfactory.io to scaffold it out, selecting the Field Types component and installed via Composer but struggling from there.

How do I define the list of icons that should show in the dropdown field?
How do I get that to then reference the correct SVG file?
How do I output that properly in my templates with {{ entry.icon }}?

I realise that's literally asking how to build this thing  but I would appreciate some guidance!
What you are describing sounds a lot like the Position Fieldtype. I would look at that plugin for guidance.