If I was going to make a ritual system for D&D I'd first determine that all rituals are a 'source' unto themselves, so there are no 'divine' rituals or 'arcane' rituals, they're all just 'ritual magic'.
Secondly I'd make it so that anyone can cast a ritual, with or without the ritual casting feat. The only requirement would be that you would need to be of the required tier to be able to cast it.
Thirdly there would be at least three tiers of each ritual, heroic, paragon & epic. Rituals would not have levels. Each tier would require a more expensive component and the expense of the component would be commensurate with the power of the ritual, ie. a heroic Knock ritual would require a silver-wrought key of 1 gold value and would open a single lock (non-magical) that the caster was touching at the time of casting, the paragon version would require a gold-wrought key of 100 gold value and would open all locks (non-magical) within a 3 square blast, and the epic version would require a platinum-wrought key of 1,000gp value but would open all locks (non-magical) within a single structure, such as a castle.
Fourthly, there would still be a ritual casting feat, however it's purpose would be to either make rituals more powerful or reduce/negate the cost of the rituals. So the above heroic version of knock would cost nothing instead of 1gp a pop, the paragon version might only cost 10gp and the epic version 100gp.
Fifthly, I would have a fourth tier called "Miracles". These are essentially world-shaking rituals that go beyond the scope of the three tiers and encompass any incredibly powerful ritual. These would be handled in much the same way as artefacts.
And finally, sixthly, I would have a level of rituals below heroic tier called "Knacks". These are castable only by people with the ritual casting feat and who are trained in Arcana. They are very, very, very minor things, much like cantrips only with slightly greater effect and slightly more time/effort required. A for instance would be a Knack version of the Knock spell that enabled the caster to pick a lock using their Arcana score, however it would have a higher DC than Thievery/Lockpicking.