The problem with "inventing" an algorithm with the purpose of generating a private key is that it is difficult to measure how much entropy (security) your private key really has.
Well, i think the chances of someone writing down a random private key that matched my private key would be higher than them being able to design an algorithm that generated my private key given some input. Much higher.
If you can think of an algorithm, there is no reason why someone else couldn't think of a similar algorithm.
Not necessarily. There's no reason to think that is the case in general.
Although my recommendation is to create a seed that has 256 bits of entropy, if you insist on creating a brain wallet with low amounts of entropy, I would suggest using an algorithm that is computationally inefficient.
That certainly is one possible feature such an algorithm could have.
Obviously, this assumes that new technology will not be invented that can go from 'brain wallet' phrase to private key more efficiently in the future.
yeah that's not an issue when the algorithm being used to "go from 'brain wallet' phrase to private key" is not known.
they will have to find some other way to crack the bitcoin address. and that's something that all bitcoin addresses would be susceptible to no matter how they were created.