Curious about how you are going to house-rule that;
Druids & clerics able to ritual cast any spell on their entire list that is a ritual whether prepared or not, and wizards & bards only able to ritual cast spells in their spellbook (for wizards) or that they know (for bards) - or everyone able to ritual cast any spell that is a ritual and is on their spell list? Or something else?
Clerics: (1) Detect Poison/Disease, Purify Food/Drink, (2) Augury, Gentle Repose, Silence, (3) Meld into Stone, Water Walk, (4) Divination, (5) Commune, (6) Forbiddance
Druids: (1) Detect Poison/Disease, Purify Food/Drink, Speak w/Animals, (2) Animal Messenger, Beast Sense, Locate Animals/Plants, (3) Meld into Stone, Water Breathing, Water Walk (5) Commune with Nature
Wizards: (1) Alarm, Comprehend Languages, Identify, Illusory Script, Tenser's Floating Disc, Unseen Servant, (2) Gentle Repose, Magic Mouth, (3) Leomund's Tiny Hut, Phantom Steed, Water Breathing, (5) Contact Other Plane, Rary's Telepathic Bond, (6) Drawmij's Instant Summons
All: (1) Detect Magic, (3) Feign Death
There's your full list of ritual magic, as far as the PHB is concerned. Practically nothing I would think of as a "ritual-able" spell has a ritual tag in 5e.
Yeah. Sooo, not really seeing too much unbalancing badness here.
I figure it'll be:
Clerics/Druids/Divine folks can cast any spell of a level they can cast with the ritual tag, whether or not the spell is prepared or if they have an available slot.
Wizards/Arcane folks can cast any spell they have in their spellbook/known or otherwise have a written form of with the ritual tag, whether or not it is prepared or they have a spell slot available. Also, to maintain the wizard's position as "ritual-caster extraordinaire," as long as they have the ritual-tagged spell in some written form/instruction, be it some scroll, someone else's spell book, etc..., it doesn't have to be a spell level they can normally cast/use.