Stormborn
Explorer
Since Shadowdragon wasn't really asking for a debate about the merits of the spellcasting classes v. other classes at high levels I wont comment on what I think about that.
If you want to keep the standard CR (a difficult as it is at higher levels to figure) simply require that after level X (wherever you think the power creep is too much) simply require them to take a level in a nonspellcasting class every other level. Fighter and Rogue offer some good choices but Expert is actually a viable way to bost both HD (IIRC) and skills and is very adaptive. If you feel it is too weak for a PC class, throw in an extra feat at every other level of Expert.
If you want to keep the standard CR (a difficult as it is at higher levels to figure) simply require that after level X (wherever you think the power creep is too much) simply require them to take a level in a nonspellcasting class every other level. Fighter and Rogue offer some good choices but Expert is actually a viable way to bost both HD (IIRC) and skills and is very adaptive. If you feel it is too weak for a PC class, throw in an extra feat at every other level of Expert.