XE6 is about having rules that allow characters to "pop the cap" at least up to 8th level power. So I was thinking about each level as a 4-part feat chain, like this:
Seventh-Level Advancement 1
Choose a class. You gain a hit die and your character level and class level raise as if you were one level higher in that class. You gain no other benefits of levelling in that class.
Special: The class a character uses for Seventh-Level Advancement 1 must be used for Seventh-Level Advancement 2, 3, and 4.
Seventh-Level Advancement 2
Prereq: Seventh-Level Advancement 1
For the class you chose for Seventh-Level Advancement 1, you gain any increased BAB and Saves as if you had taken a level in that class. You gain no other benefits of levelling in that class.
Special: The class a character uses for Seventh-Level Advancement 1 must be used for Seventh-Level Advancement 2, 3, and 4.
Seventh-Level Advancement 3
Prereq: Seventh-Level Advancement 2
For the class you chose for Seventh-Level Advancement 1, you gain any increased skill ranks and ranks available as if you had taken a level in that class. You gain no other benefits of levelling in that class.
Special: The class a character uses for Seventh-Level Advancement 1 must be used for Seventh-Level Advancement 2, 3, and 4.
Seventh-Level Advancement 4
Prereq: Seventh-Level Advancement 3
For the class you chose for Seventh-Level Advancement 1, you gain any benefit of taking a level in that class that you have not already received from Seventh-Level Advancement 1, 2, or 3. You gain no other benefits of levelling in that class.
Special: The class a character uses for Seventh-Level Advancement 1 must be used for Seventh-Level Advancement 2, 3, and 4.