Drupal 8.5.0 will come with a new experimental module, Layout Builder.
What is layout builder? Well its basically panelizer in core.
Watch the screencast for a quick run down of how to setup, configure and use it.
Love the video, this functionality feels essential to moving away from some of the d7 sites i've built before.. excellent. Quick question though- do you envisage it working with editable fields (like panels did)?
Great to see this project a really good page builder is badly needed for Drupal - looks like a very good start, well done Lee.
Not sure if you are familiar with the layout builder and visual composer build by NikaDevs (a theme company) but you could do a lot worse then having a look at their approach, it's a very good page builder - which they ave on all their themes.
I also suggest you take a look at Glazed Builder, they have a really nice visual approach.