Nothing, yeah no, seriously. nothing will happen when all the Bitcoins are mined. The price of Bitcoin is determined by current demand to transfer value and expected future demand. The only way this will change is if a constant stable amount of value were continuously being transferred through the system, which would require significant real use. As yet real usage is pretty insignifcant and most transactions are probably speculative. Anyway, we cant predict the future.
we cant mate, but if we are positive thinkers we will believed that after everything was mined the real purpose of bitcoin will be served as a currency to lessen
transaction period, bitcoin will continue just like real fiat where circulation are the big factors of growth.