Thanks for the great idea! I have a question: how do you expect such fields to behave in reporting? Or is it needed only for object details pages? 

I'm thrilled to let you know that this functionality is now available, and you can make the External Lookup field accept multiple values:

Thanks for the feedback, William! May I ask you to provide more details on this feedback? What do you mean by a new object type? I'm happy to jump to a call to discuss this. 

Hi Doug, William has a great response above detailing how the cascading works using this new field type. Thanks,Gagik

Hi Mylah, I'm happy to let you know that we have added the External Lookup field type to custom forms that might cover your needs. It is already in preview and will be released with the upcoming release. Here you can find more details on that. ThanksGagik

Hi Mylah, I'm happy to let you know that we have added the External Lookup field type to custom forms that might cover your needs. It is already in preview and will be release with the upcoming release. Here you can find more details on that. ThanksGagik