DISCLAIMER: Expressed views on this blog are my own.
I've been looking for a way to pass properties and/or attributes to a slot element through to the target element that replaces the slot, but that isn't supported behavior in the W3c spec. I spent a few days trying to google for a solution heavily, but instead landed on the simplest solution. I use lit (http://www.lit.dev) for creating web components because it is perfect for any use case.
I found one idea to pass attributes and properties here: https://pierrelebrun.me/dynamic-properties-on-slots. I find it complex. I don't like the idea so I didn't bother implementing it.
The solution I figured out was to pass a property into the parent element called childRenderer: (idx, ....) => html`<child .propname=${idx}></child>` and remove the slots. Now I have full control of how to render the children again without slots. The children will render and the properties will be passed in.
Essentially, <parent items=${["bla", "blah"]} .childRenderer={(idx, item) => html`etc etc etc..`}></parent>.
It's a trick I learned during my React days, too bad I didn't remember that until now.
Again, glad to have moved away from web frameworks (no more Svelte!) to my own custom solution which I landed on in less than a day after writing about it. My heuristics are just that good. I can make changes and add files with rollup detecting changes and the changed parts gets compiled. Millisecond builds, live reload, server restarts, typescript, web components and all the flexibility I need. My dev experience is more amazing than it was with Svelte. If you want to build lasting, higher performance, low maintenance web apps then forget the all in one frameworks.