How do i know what features to leave out of the MVP?

I think you've already answered your own question. The goal of an MVP is market validation. "Does anyone even want the bare bones of my idea, or am I wasting my time?"

Until you can answer that definitively, you shouldn't build anything extra.

I'll play devil advocate to others - as I don't think you actually answered your own question in terms of the details you provided.

What's the problem being solved? Why does someone want to copy components? What do they do with it? Is not having the builder like having a nail with no hammer?

If you have something showable and can get feedback on, then that's an awesome place to start. If this starting point says it's more important to focus on this next feature or on existing features, then knowing and talking to your customers will better inform what the true MVP is (if any).

What are you trying to find out with your MVP? What are the riskiest assumptions you have that you want to validate first?

Your MVP should answer the above questions to give you enough information to decide whether to pursue it further.

I agree with @ryanvanderpol, I think you answered your question. What you have is an MVP and what you want is an Enhancement to the MVP.

When I talk with people about how to decide what to keep and take out I typically have them look at a few things.

1. what will my customer miss if this is not added? Take note this is not asking if you don't add X then they don't get X. instead its asking what how does feature X missing causes my app to feel less of. take a login form without client-side validation. you can go live with this no being there but your customer will not know if they truly typed in the correct information until AFTER the press the submit button and the server-side validation happens. Now you have to look at and see if you think this is acceptable. Most people would jump in and so NO! but what if by not having this client-side validation in place you actually learned MORE about how the form was used and then created some client-side validation that was more customized to how people use the form!?!?
2. What's the demand for what you want to add on? did a customer ask for it? has a petition been signed for it? are people leaving your product because you don't have it? engineers and product people can come up with ideas all the time but the most important ideas are those generated by your community. Solve their problems and pains instead of creating new features they won't use and potentially creating new problems for them!
3. Will adding this gives us a competitive edge on launch day? are we prepared to support the be features? can we handle an explosion in growth because of the new feature? Can we handle a mass exodus of users because of the new feature? how are we altering the way people use our app with the addition of this new feature.

Lots to digest but that's the point. Adding something NEW should be difficult, and painful on some metaphysical level so that people are conscious of their decisions rather than just throwing more stuff on the pile.

Best of luck to you! the widget sounds cool!!