Can bitcoin be forked in such way that the monetary base increase? Can it be forked into a new coin, such that if i had X bitcoins i will still have X coins in the new crypto-coin, but the monetary base will be 50M coins for example?
Yes. It's possible to do that in a not backward compatible fork (hardfork).

Assumed that the new currency would be bitcoin50 (B50):

You could copy the whole Bitcoin-blockchain (as in every hardfork) but only modify (increase) the future block rewards so that there are around 50,000,000 B50`s available when the reward is 0 in ~100 years.