I feel Bitcoin will be a lot centralized.

Bitcoin will have a face to the founder.

If centralized, we have to doubt privacy and security, in blockchain and investment related terms.

This is not true. For a long time, most people ignored Bitcoin. If what you say was true, a lot of people would have rushed to Bitcoin as soon as they first heard of it. Generally very few people still care or understand the flaw of the central banking system. If bitcoin was not a decentralized cryptocurrency with sound cryptography, it would have died a long time ago so you are very wrong here.

We can talk about these ideas in theory, but it is impossible to recreate the environment that led to the creation and continued existence of Bitcoin. There will never be anything similar to Bitcoin, where decentralization emerged naturally to the point that it is at now. All altcoin creators fail to understand this, or maybe they don't care because they just want to get rich.

Please! we can't be left with philosophical doubts about what would happen if... unless we're in the experimental phase, hence Bitcoin isn't; it's a fact that's being implemented right now. So, assuming what would happen if this or that were done only leads to falling into the dilemma: "what would it have been like if I had met you before," as a popular song goes.

You could probably replace secp256k1 with technology that is more quantum resistant. Of course it doesn’t come without sacrifices. There will be some performances deficit as keys may be too large which makes transactions slow. It wouldn’t be compatible either with a lot of existing wallets we are used to

My question is how would it do in the marker? It can be marketed as future bitcoin ir better bitcoin addressing the concerns of many but will it be as successful as bitcoin? Maybe, maybe not. The reimagined bitcoin would have to start from scratch again while bitcoin remains to keep surviving. A reimagined bitcoin will just be another altcoin and not an exact copy of bitcoin.

I find it hard to imagine something that has already been invented, in other words, when we already have some kind of reference point to compare. It’s when something doesn’t exist that the situation is truly different. From other perspectives, I think we have altcoins for that, but even with its early issues, Bitcoin was something unprecedented and unimaginable at the time. It’s like discovering fire or inventing the wheel, you just can’t picture creating something like that from a modern perspective. I’m not sure if I made my point very clearly.

Do you mean what if Bitcoin is copied or recreated?

If that is what you mean, then my answer is no way. Because I have seen many projects that even tried to be a competitor or replacement for Bitcoin, but in the end they failed, and that also happened to Ethereum. Come on, even for Ethereum no one has been able to try to copy its success, let alone Bitcoin, right? Of course it is.

Well, if that's what he mean same features or it's just a complete clone of BTC I'm sure no one would use it or buy it because there is no unique features why would I buy this new coin if I already get the feature on BTC?

Look at other coins out there with similar roadmap they end up become scam or become useless. It would never be success same as Bitcoin unless it has a unique feature that people wants and have trusted developers.

Modern cryptography is not vulnerable to quantum computers too? Imo, Bitcoin is better as it is and doesn't need any flavoring or anything to make it better or spicy. We have altcoins for all of that. As a matter of fact,  it's because of a "better bitcoin" that we have Ethereum and over 100K worth of altcoins with at least 99% being useless.

If Bitcoin's success refers to the financial crisis that occurred in 2008, then shouldn't Bitcoin boom and go viral immediately when it first appeared? Why did it have to wait a decade for Bitcoin to be widely known outside the crypto community? As far as I know, Satoshi created Bitcoin because of the 2008 world crisis. But along the way, the decentralized nature of Bitcoin and its creator who did not appear to the public made Bitcoin one of the most transparent financial systems in the world because it could not be intervened by banks or governments. I still remember the hype that happened in 2017. At that time, people bought Bitcoin not because it was the answer to the 2008 financial crisis or because it was decentralized. They bought Bitcoin because they didn't want to miss the train. But over time, people began to realize the added value of Bitcoin so that Bitcoin became what we see today.

If Bitcoin was reimagined today, I guess it wouldn't be as decentralized as it is right now because reimagining means governance and central decisions taken on the coin. But lots of things would change. I think that today, reimagined Bitcoin would be quantum resistant. I think that it would have more energy-efficient consensus than Proof Of Work is. Transactions would be either more anonymous or they would be left the way it is now but I'm sure that the fees would be much lower and the block size would double after every halving or it would be flexible, the whole system would be designed in a way to prevent miners and spammers to abuse transaction fees and to get rid of unwanted block size to make running a Bitcoin node easier and accessible for many people.

I'm sure that lots of things would change but it's hard for me to speak better about it because it requires knowledge of programming and cryptography.