Currently, Bitcoin is quite limited in regards to its cryptography and reaching its true decentralization potential. One of my aims is to change all that. Bitcoin has the problem of an increasingly large carbon footprint, potential threats to it being breakable, and specialized brute force attacks that are becoming increasingly complex.
Two of the biggest threats I believe is actual solving of the P vs. NP or Post-Quantum cyber attacks. However, Bitcoin's cryptography hasn't radically changed in regards to these potential threats, and will need to radically change in order to be sustainable in the future. Ofcourse, this is just my opinion. However, It think it is a quite valid one.
I also think that limited the mining of Bitcoin to ASIC specific hardware and not expanding on usages for "Proof of Work" is a critical flaw that Bitcoin has. I believe there are ways to expand mining capabilities without disenfranchising current users, and already talked about this in a draft.
That said, I still think Bitcoin is great the way that it is designed. The brilliance of the system as a whole is the reason Bitcoin is thriving today. It is also likely the reason it seems to be ranking supreme. However, I think BTC can do better. I think the cryptography can become more complex, the mining can become more adaptable, and the efficiency can greatly improve.
It is still quite early for Bitcoin, even after all these years. Bitcoin's market cap have only recently reached half of what the government just printed out of thin air. The future ahead if done right, can be quite exciting.