I would like to see "Ritual" spells in a separate design space from slot spells. The slot spells are intended to spend slots to cast them spontaneously, especially for combat.
In contrast to a spell, a "ritual" would say a DC and which skill check is necessary to perform it, would list the effect that happens when successful, and what happens if the check fails and the magic goes awry. Depending on the particular ritual, any effect might be possible. Any class can attempt to perform a ritual via the skill check. A caster might spend a slot to guarantee a success, but would probably do it by skill check anyway to conserve slots for combat. Each ritual can have any requirement, a certain minimal level, any amount of performance time, be done at specific astronomical events, require any special ingredient, costly gp ingredient, certain accomplishments, certain tasks, whatever. Each ritual stand on its own and describes its requirements and effects. Most of the useless spells, like Hallucinatory Terrain, would be rituals instead, for that one time when it might find use. Some rituals would be useful, others obscure. Rituals would be a kind of magic item treasure. With such rituals, D&D is able to actualize most of the stories about magic in popculture and folkbelief, especially where anyone can try to perform one, and it might go wrong with bad magical consequence.
