forking bitcoin on the same blockchain is going to cause many problems
Not raising the blockchain limit to a reasonable size will also cause many problems. I personally believe the long term effects of 1MB blocks far outweighs any fragmentation risks with a Xt hard fork, assuming 75% miner supermajority.
There is no need to let 'alt coins' take any transactions from bitcoin. The analogy of Bitcoin being gold is misguided, as bitcoin isn't just gold. It can be gold, silver, copper, and diamond, at the same time.