Forgive me, I'm still getting my hands wet with utilizing the existing ritual system, so please ignore me if I'm already calling out spells that are already rituals; my main goal is to expand the list without it becoming too powerful.
Yup, that's fair.
That's part of the reason for the tight clamp - attunement + 1/day. I'm hoping to start with higher restrictions and loosen them, rather than needing the reverse.
I would avoid the phrase "attune" simply because "attunement" is used to describe the limit on the number of magic items that you can use at any one time. I would just say "prepared rituals" or "selected ritual spells".
Also, since I don't run campaigns past about 9th-12th level (in the 30 years I've played), I'm not overly familiar with higher level spells in actual play (especially for this version). Part of posting here was to get some comments on what spells added to the list were or were not appropriate.
Honestly, with the requirements you've set (prepare only proficiency bonus rituals, 24 hours to prepare one ritual (or was this just restating the 1/day thing?), each ritual only once a day) I'm not convinced there are
any spells below 6th level that would not be acceptable as a ritual 1/day.
Polymorph is probably one that stands out to me that I would think about, or
planar binding or similar magic. Anything that lasts longer than the ritual's casting time (even with your extended time) or is instantaneous (i.e., permanent) is at least a little suspicious.
Conversely, I'm not sure I would allow any 6th level spells to become rituals. There are only two 6th level spells that are currently rituals. It would really depend on the spell. I can't think of any spells of 7th level or higher that I
would allow to be rituals (there are
zero ritual spells of 7th level or higher currently). The game very deliberately limits spellcasters to a
very small number of 6th, 7th, 8th, and 9th level spell slots. You must be level 19 to have two 6th level spell slots, and you must be level 20 to have two 7th level spell slots. You never get more than one 8th or 9th level spell slot each.
If you look at Warlock, you'll notice that 6th and higher level spells are cast through Mystic Arcanum instead of Pact Magic, and you get different rules. In reality, all spellcasters have this limitation, it's just better hidden. It's a very deliberate design decision. Having access to 6th-9th level spells is itself a major class feature.
Either way, I think this adds a significant amount of flexibility and power to most spellcasting classes. I'd personally rather have the ability to cast
detect magic when I needed it, but this is more powerful.
As for spells such as Charm, those are more villian options - using a ritual on a captured enemy to bend them to your will to perform some required task. Less so something I would expect an actual PC might use, but possible.
I would not even worry about this. If an NPC needs a spell out of combat, they should just have it prepared and available to cast. Especially a 1st level spell. NPCs are not really obligated to follow the same rules as the PCs. It's not cheating. It's DM's perogative to keep the game moving. You're the man behind the curtain; don't feel bad about pulling a few tricks. Unless you're doing something like casting charm on 100 people in a row, it's not worth bothering with the details (and then you could use
eyes of charming).