All things regarding our IDE, including usability issues, consistency problems, feature ideas, and anything that will make you a faster developer.
If it relates to behavior on the browser, this is the place: Themes, UI Flows, Screens, Entry Points, Widgets, CSS, React, Ajax Refresh, EPA Taskbox, translations, etc
If it relates to behavior on the server, this is the place: BPT, Entities & Entity Diagrams, security, emails, multi-tenancy, site properties, bootstrap from Excel, and Authentication.
Every suggestion related to Service Center: Application management, monitoring, logging, etc
All ideas related to connecting to other systems, like REST, SOAP, SAP, extensions, or Integration Studio belong to this category.
If it relates to querying the database, like aggregates, advanced queries, preview data, test queries, this is the category.