Since the concept of Prestige Classes haven't really been explained, i'll try to do so.
the entire concept is specialization, and it can be visualized like this:
you have a Rogue. He can sneak, hide, move silently and stab people. Right now he is a Jack of all trades but master of none. So you use a prestige class to make him exceptional in 1 area and less so in others.
You do this through Assassin. He can now stab people really well, hide and move silently. But he didnt improve in the Sneaking or the Stealing part. However he is alot stronger in combat than before, however he wont get the bonuses from getting a level up in Rogue, as he chose Assassin instead.
the reason this cant really be implemented in Median is because its already there. You start off with a Barbarian. Then you "Prestige him" by choosing to use Melee, Ranged or summons. Then you further "prestige" him by choosing an über, that compliments the character. The Paladin is also a good example, since Holy and Unholy straight up locks out the other tree.
If this was to work in Median, you would have to lock out some progression somewhere. IE: If you choose Skill A at lvl 12, you cant choose Skill B and so on. That way you can get a specialized character that cant excel everywhere, because of the progression style. I Highly doubt it would be effective, but its a possibility.
i hope this made sense