actual a class can have more then 20 levels see the beyond 20 rule in core rule book, it even tells how to figure out. you just don't get anything new, this is why multi class is suggested as easier option, you still get new things. even multiclass is not a good option because all the new things your get are actual old things that are more then likely not going to be useful at that level of play any way.
"Experience Points
To gain a level beyond 20th, a character must double the experience points needed to achieve the previous level. Thus, assuming the medium XP progression, a 20th-level character needs 2,100,000 XP to become 21st level, since he needed 1,050,000 XP to reach 20th level from 19th. He'd then need 4,200,000 XP to reach 22nd level, 8,400,000 XP to reach 23rd, and so on.
Scaling Powers
Hit dice, base attack bonuses, and saving throws continue to increase at the same rate beyond 20th level, as appropriate for the class in question. Note that no character can have more than 4 attacks based on its base attack bonus. Note also that, before long, the difference between good saving throws and poor saving throws becomes awkwardly large—the further you get from 20th level, the more noticeable this difference grows, and for high-level characters, bolstering their poor saving throws should become increasingly important. Class abilities that have a set, increasing rate, such as a barbarian's damage reduction, a fighter's bonus feats and weapon training, a paladin's smite evil, or a rogue's sneak attack continue to progress at the appropriate rate.
Spells
A spellcaster's caster level continues to increase by one for each level beyond 20th level. Every odd-numbered level, a spellcaster gains access to a new level of spell one above his previous maximum level, gaining one spell slot in that new level. These spell slots can be used to prepare or cast spells adjusted by metamagic feats or any known spell of lower levels. Every even-numbered level, a spellcaster gains additional spell slots equal to the highest level spell he can currently cast. He can split these new slots any way he wants among the slots he currently has access to.
For example, a 21st-level wizard gains a single 10th-level spell slot, in which he can prepare any spell of level 1st through 9th, or in which he can prepare a metamagic spell that results in an effective spell level of 10 (such as extended summon monster IX, or quickened disintegrate). At 22nd level he gains 10 spell-levels' worth of new spell slots, and can gain 10 1st-level spells per day, two 5th-level spells per day, one 7th-level and one 3rd-level spell per day, or one more 10th-level spell per day. At 23rd level, he gains a single 11th-level spell slot, and so on.
Spellcasters who have a limited number of spells known (such as bards and sorcerers) can opt out of the benefits they gain (either a new level of spells or a number of spell slots) for that level and in exchange learn two more spells of any level they can currently cast.
You might want to further adjust the rate of spell level gain for classes (like paladins and rangers) who gain spells more slowly than more dedicated spellcaster classes."
best option and maybe you should suggest to gm is use 20 level and the mythic stuff after, it plays as little nicer with the system as a whole.
