In discussion, we thought of three main things which need adjusting:
- DCs grow rapidly from one to twenty one, then stagnate until forty.
- Low level spells are essentially shut out for DC casting, as a result of the PnP formula. Higher level spells already outperform low level spells, so this puts level 9s leaps and bounds in front of level 5s.
- Casters already have spell penetration and numerical effects tied to caster level, so we won't tie DC to caster level.
So, for a "traditional" DC 48 caster, you're looking at DC 54. Depending on how the saves bonus plays out, the "5 +" part could end up being +6 or +7 to the DC, to ensure that DC casters are ~the same as before in terms of balance.
There were also two or three things which were more "Oh, and this would be cool too":
- Wizards should probably get +1 DC for their specialised school.
- Druids might get +2 DC for being outside.
- Perhaps 40 caster gets another point of DC to enable DC casters to sacrifice discipline without sacrificing DC.