Which should we work on as the unsolved problems in cryptocurrency and why?
Scalability - Increasing transaction processing capacity and speeds on blockchains to support wider adoption. Issues around block size limits, transition to proof-of-stake, sharding, layer 2 solutions.
Privacy - Enhancing privacy protections for users on public blockchains while meeting regulations. Zero knowledge proofs, confidential transactions, mixers/tumblers.
Security - Preventing hacking attacks, thefts, fraud on blockchains themselves as adoption increases. Consensus protocols, key management, formal verification.
Governance - Determining the best mechanisms for protocol upgrades, conflict resolution, user representation for decentralized networks. On-chain voting, off-chain coordination.
Regulation - Clarifying how cryptocurrencies fit into existing regulatory frameworks around securities, commodities, payments. KYC/AML compliance for exchanges.
Volatility - Reducing price swings to be more usable as currency/payments. Stablecoins, reserve-backed currencies, tokenomics.
Energy - Developing sustainable alternatives to energy-intensive proof-of-work mining ascriticism rises. Proof-of-stake, off-chain computing.
Usability - Improving user experiences and interfaces for transacting and managing assets and wallets. Simplified addresses, fiat on-ramps.
Interoperability - Allowing inter-blockchain communication and exchange of value. Cross-chain protocols, atomic swaps, blockchain bridges.
Others - We welcome your own ideas/pain points
Scalability - that happens through second layer networks, side networks, and stuff like that. Doesn't happen on-chain, except in small doses like Segwit. And it certainly doesn't happen by ruining Bitcoin with PoS.
Security - Bitcoin is already the most secure network in human history, by far. Things could certainly be done in terms of UX security like with key management.
Governance - Bitcoin already has a way to signal voting on protocol upgrades. I suppose it would be nice to have some sort of unofficial mining, node-runners, user, and code contributer organizations that were open and discussed issues, but that would be a community creation not a development issue.
Regulation - that's up to governments.
Volatiliy - that is a market feature, it is going down over the years and will continue to do so as Bitcoin grows, but it will remain fairly high until Bitcoin adoption matures to a significant degree and until the halvings stop causing huge hype cycles (or seen another way, until crashes stop causing people to get out of Bitcoin and assume it's dead).
Energy - Bitcoin already has the best consensus mechanism in Proof of Work, nobody would ever want to change that. It has huge benefits for Bitcoin and for society and the energy industry. The hate on mining by the public is mostly just a lack of education and optics thing, not a problem with actual mining.
Usability - For sure usability can be improved. On-ramps, wallet UX, user UX safeguards to not make a mistake when sending money, key management solutions, etc are all problems that can still be worked on from a usability standpoint.
Interoperability - Some bridges and whatnot already exist between Bitcoin and Crypto. I don't think this is terribly important, at least not yet, as nothing besides Ethereum has shown any staying power. It makes a lot more sense for people to trade their Crypto for Bitcoin, than try to keep a link to their altcoin by temporarily bridging over to Bitcoin. Though I suppose perhaps bridging wouldn't count as a tax event. Perhaps if Crypto ever takes off and gets some popular apps and some Crypto tokens start showing staying power this issue will become more important.