Bitcoin leads as the first successful crytocurrency in the world. Its successors are altcoins. Altcoins have been developing for different deficiency in Bitcoin or with innovative ideas.
Bitcoin has not privacy. So this brings new innovative crytocurrency ideas like Anoncoin with extensions for I2P or Tor anonymous networks, Bytecoin with ring signature technology, Dash with mixing services, Zcash with cryptographic protocols like zero-knowledge proofs. But these altcoins do not have market capitalizations as large as Bitcoin and price as high as Bitcoin price. Bitcoin has longer history than altcoins.
So, it has more investors and users as normal. But it is foreseen that Zcash will find a high majority of users because of its fully private transaction property. Although, some communities have criticisms that untraceable transactions cause money laundering, tax evasion and higher trading in black markets with illegal drugs, weapons, human trafficking, stolen credit cards, fake driver licenses, child pornography. These criticisms are valid for all anonymous altcoins.
The second must improvement for Bitcoin is scalability. As its first years, scalability was not a big deal because of its small size blockchain and network size. But with time scalability debates raised. Bitcoin community and developers started to improve different proposals like BIP family and innovative blockchain ideas like Bitcoin-NG. But it has some security concerns. Also for scalability, some blockchain pruning developments have been studied by researchers. There is a tradeoff between security and scalability in the big picture. As we explained in scalability chapter, some scalability improvement proposals have pitfalls in terms of security. Developers and community of Bitcoin settled on a consensus about scalability and we will experience in future 1-2 years how it will work for security and scalability.