If that's too broad, then you could go "every odd" level like they used to be (the next tier of spells being at PC levels 3rd, 5th, 7th, etc,)...but at that point, you might as well just go back to "Spell Levels" being a different separate system than caster/PC level.
In the Arcana Unearthed design diaries, Monte Cook talked about the major power jumps in traditional D&D magic. I agree entirely with where he put them for arcane magic: 1st level spells, 3rd level spells, 6th level spells, and then the big stuff that makes up the more overpowered options at the end of the scale. Having just reviewed all the spells in the Rules Cyclopedia in the last few weeks, I saw nothing that would disabuse me of that notion.
The line on individual spells by level gets rather fuzzy between 6th and 9th, as some spells around 6th are rather earth-shattering, while some of the 8th or 9th ones are well short of what some of the other spells can do. And of course, there are several 2nd, 4th, and 5th level spells that could easily go either way. I'd move traditional
invisibility up into the second tier, for example.
When I mentioned "apprentice", I had cantrips in mind as the zero level stuff, but no reason why that can't be lumped into the first tier, especially if they go through with making the first 3 to 5 levels about putting together a strong character.
Hmm, I wasn't even thinking of 4E tiers when I wrote the above, but that is almost a match. If you divide the 4E 30 levels into 4 tiers, as some of us have discussed before, you can name the spells by the tiers:
- Apprentice Tier - character levels 1-5, traditional spell levels 0-1, some 2s.
- Heroic Tier - character levels 6-15, traditional spell levels 3-5, remaining 2s, a few 6s.
- Paragon Tier - character levels 16-25, traditional spell levels 6-7, remaining 5s, a few 8s.
- Epic Tier - character levels 26-30, traditional spell level 9, remaining 8s, any really cosmic stuff.
Or if you want to compress it to 20 character levels to end up with something closer to the 11th level full caster getting 6th level abilities, same principles. Character level tier breaks at 1-3, 4-10, 11-17, 18-20, is pretty darn close to the actual time full casters picked up the respective power jumps. A 4th level caster is starting to dabble in "heroic" magic, at its lowest power.
Makes me want to start a new topic to see if any existing spells would be hard to place in such a scheme.
Edit: I had not read Frostmarrow's last post when I first wrote the above.