In terms of mining scalability, bitcoin will end up with the heart of the system in the hands of a few individuals, and that's one of the biggest problems with it. I have started https://vialcoin.com in a humble attemp to fix that. It's still a work in progress and I value greatly any input or help in my mission to create a true cryptocurrency for the masses.
Check out this post from 4 years ago ;)
https://plus.google.com/u/0/b/110629124582721325829/+VialCoin/posts/a4XzpEfGVBa