Is logic to show / hide fields part of data or template in view?

The goal is to reduce the amount of logic that is embedded in sections right now - using visible when on property will be supported.

Can developers create their own reusable templates?

You can create your own template right now for sections - Templates in the React based UI architecture will have to be built as React components. If a template doesn't currently exist for a desired layout is it safe to create our own template. It is better to create a reusable design template used throughout your app than creating one off sections that are not consistent. You can create your own design templates.