Regarding the Bingo Book idea

Bingo Book consists in keeping the cap 20k but adding a 2nd cap that would be at lv 30k. The only way to level up from the base 20k is to have your name written in the bingo book and complete dungeons and/or hunt other players who are also in bingo book.

Your name can only be entered in the bingo book if your a certain rank or level, lets say S rank or have completed a certain kind of secret requeriment.

After this point you could unlock secret clan jutsus and reach some kind of "Madara Level".

Each level after the 20k cap would give a single Stat Point, meaning that you would gain an extra 10k points.

EDIT:
Such mechanic would do wonders to increment the PVP and PVE values of the game since there would be more rewards for both of those aspects.

Squad system could be improved to help Dungeon cleaning as you could do it with a friend instead of solo.

Regarding the 30k cap thought, the reason for this is that putting the level cap too high would kind of unballance the jutsus. A soft +10k levels could in some points help improve some builds and even create new possibilities.