If birthsigns are developed over time, it's no longer birthsigns. I like the concept said above, but it shouldn't be called birthsigns then.
Not sure. I've always felt that while everyone in Tamriel is born with the potential to harness their birthsign's power, they may not truly realize it or be capable of doing so. Do you really think that lowly carpenter who has never adventured a day in his life would be going around using the power of The Serpent to poison his enemies? Doesn't seem quite right.
Racial powers, I feel, are things that everybody is aware they possess.
But birthsigns are something, depending on your actions, you may not fully realize until later on in life - just as how we don't know we're the Dovahkiin nor have the power to use Thu'um until the events of Skyrim. Personally I think the average citizen of Tamriel doesn't walk around going,
"yeah, I was born under the steed, I'm way faster than you guys."But I know what you mean in a sense - sudden acquisition of a power without any foreshadowing would be kind of gimmicky. Then again, aren't perks doing that in a way?