I see two aspects to rituals that I'd like to explore.
The first is a way to involve multiple casters, possibly of disparate disciplines, in a single magical effort. The "Three witches" part of "Enter three witches."
The second is using rituals as ways to modify spell results.
The Red Wizards of Thay prestige class from the DMG has some rules for sharing power through a ritual. Theirs has a group feeding spell power to the leader, allowing that leader to prepare spells with metamagic feats without having to personally use higher level spells slots. That's as close to group rituals as I've seen in D&D.
In our campaign our group was tasked with sealing some portals to elemental planes (presuming you include the Negative Material Plane as "Elemental Death". )
It was agreed that this took a ritual, since there isn't actually a spell that can do it. We played it through without any actual rules for it. That's at least part of what got me thinking about this.
As we played out these scenes, and as we've discussed things here, I've come up with some rough ideas. Let me give our "Seal the gate" ritual's rationale.
The base spell we're working with is Magic Circle, the area version of Protection from Evil/Low/Good/Chaos. That spell prevents entry by charmed, enchanted, summoned or conjured creatures. The ritual twists the spell slightly, so it prevents entry by all extra-planar creatures, not just those called by way of a Summoning. In trade, it loses the Deflection bonus to AC the spell normally provides, as well as the protection from certain mental influences it gives.
Slight tangent: One standard we've established has to do with casting spells through portals or gates. You pretty much can't unless ste spell says it can affect across planes. It has to do with spell range; How far away is that other plane, in terms of feet or inches? Get the idea. But we've established that area effects can expand through the gate or portal.
Add this concept to the way we're handling that ritual. If the Circle remained only on one side of that gate then things from that side could go through, but nothing could come back the other way. If the area of the Circle expands through the gate, however, then it sets up it's own mirror image on the other side. Nothing can enter either Circle from the other side. The Gate is still technically there, but nothing can access it.
Now applying those concepts on a broader basis, a Ritual allows a spell effect to be modified, and/or to have meta-magic effects added. The leader of the ritual casts the Primary spell to be used as the foundation. Other casters supply related or supplemental spells selected in advance because they carry at least some of the traits desired in the new spell effect. Additional spells can be used to power the addition of meta-magic effects. For example, if you want to empower the effect, you need two spell levels, since that normally uses a spell slot two higher than the base spell. (That's straight from the Red Wizard rules).
Also I'd add a Ritual Caster feat to the game, with the note that Clerics get it automatically, the way Wizards get Summon Familiar.
Casters without the Feat can still participate in ritual casting, can even lead the casting, but they're limited. The highest level spell that can contribute is one of half their top rank (rounded down). Casters with the Feat can contribute up to their top spells. In all cases Contributed spells should be of the same school (Abjuration, Conjuration etc.) as the Primary spell used in the ritual.
And yes, while some rituals can become standard, all rituals need to be cleared with the DM, both in what they can do and in what they cost. Cost is something not mentioned. There should be one, either in terms of Exp or materials or both. Want the spell effect to be permanent, as our Close the Gate ritual had to be? Exp is contributed by all participants. The total should be 500 Exp per spell level, which is the basic framework used by the Permanency spell.
Rituals take at least an hour.
So why do Clerics get Ritual Caster for free? Well, Wizards get several bonus feats, directly or indirectly. Summon Familiar gives the Wiz' the effect of Alertness. Scribe Scroll is specific to the flavor of Wizards. To my way of thinking pretty much every religious ceremony comes steeped in ceremony and ritual. It seemed like a natural.
Anyway, that's what I'm thinking and where I'm going. Maybe.
It's also possible that it's 11:00 pm, I'm tired, and my keyboard overfloweth.
Yeah, maybe it's that last one....