During runaway inflation of fiat currency, when common items start to cost thousands and millions of <insert currency name>, they simply redefine the currency. So a million old X = 1 new X. They could do that with bitcoins too, except in the opposite direction. Currently, bitcoins are divisible to .00000001 (10^-8), which is called a Satoshi, but that's not a hard limit. 1 Satoshi could be redefined as 1 bitcoin and re-partitioned to 10^-8. In that case, someone holding 10 BTC today would hold a billion of the new bitcoins.