Hi there @Elli,

Thanks for being a member of our Community! I hope I can provide some info regarding this idea.

Our back-in-stock flow will not work for this use-case. This does require a specific variant to be restocked. If you would like to be able to notify customers when something similar is in-stock you will have to collect their information through a form.
You can have your form be any style (embed/pop-up/fly-out) and then have drop-downs for what they are interested in or add additional preference options. You can then have these profiles added to a list specifically for re-stock notifications. Their preferences will also be saved on their profile.
Now that you have their preference information, you can create segments based on these. For example if you were selling clothes, you can create a segment that looks for everyone that has a preference of size: medium. Then you can send a campaign message to that segment when a medium size is in-stock.

(Additional doc regarding how segments update can be found here)
This process does require you to manually create a segment, but will still allow you to engage your customers.

I hope this helped!
Alex