The first time a PC frenzied berserker attacks the party is the first time my character murders a PC in his sleep.
So IMHO stay away from them.
Given that your character is dumb as mud... I don't think you'll be able to qualify for many PrCs. Typically they require skill points, which you don't have. If you want to take them, you'd best get some rogue or ranger levels into you.
Having said that, rogue, ranger and fighter all make nice synergies with barbarian. You can make a fine PC just sticking with multiclass combos.
Finally - if you want a PrC, you should probably have some sort of concept in mind for your character. "Hitting stuff with a flail" doesn't really qualify - although I suppose you could try for some of the weapon-master style characters.