Perhaps there is some terminology confusion: “Concept” in RemNote refers to a “Concept Card”, a type of rem that has a bolded front and regular back that may be practised as a flashcard in either direction. Other possible “Rem Types” at the moment are Descriptor and Slot; “Powerup” is a tag that adds something on top of any rem (e.g. Header, Document, Automatically Sort); Slots may be added under any rem at all and will be suggested for any other rem that is tagged with a rem that has slot children. The queries as they are implemented on the test server have separate filters for Rem Type and for regular tags. In your usage, the only reason to turn rem into concepts is to bold them, as you can already manually create a rem that has the text “Concept” anywhere else, add slots to it, and tag other rem with it to have those slots be suggested to you.
The reason adding slots to rem types is untenable is that the types are meant to be universal and only concerned with how the rem behaves, not its contents. Tags, in contrast, can be applied selectively depending on the contents, and so can apply the appropriate slots.