There are more centralization right now in DHF and how 3 stakeholders are able alone to approve proposal

There is no way to address this problem, other than to improve the overall coin distribution.

With DHF funding, if a large stakeholder doesn't like a given proposal, there are 2 ways the large stakeholder can use their stake to keep it from being funded -- [1] don't vote for it and [2] vote for the return proposal.

With witness voting, there are innumerable ways a large stakeholder can unilaterally diminish (i.e. punish) a disfavored witness. By unvoting witnesses that the large stakeholder would otherwise support, but who already have enough funding to be in the Top 20, the large stakeholder can throw all their support behind the 30 witnesses who would (without that support) be just behind the disfavored witness, effectively pushing the disfavored witness down the list by 30 places. And, by dividing up their stake (via multiple accounts), the large stakeholder could conceivably push a disfavored witness down the list even more.

I am not saying any large stakeholders are actually doing this. My point is that the current system allows this kind of abuse, and thus should be reformed.

And in some aspects, like marketing, we surely should be more centralized (all involved in one unique strategy) because because 10 poor marketing strategies is never better than 1 good marketing approach.

Ah, yes, but wouldn't 2 good and 8 poor (relatively small) marketing strategies be better than 1 big (but poor) marketing strategy?

Good centralization is good. Bad centralization is bad. The problem occurs when the stakeholders disagree about what approach is good and what approach is bad.

Whereas witnesses are meant to represent the community, I would greatly prefer to see witnesses reaching out to community members for input and ideas rather than having them merely talking amongst themselves.

Or, having various members of the community hosting town-hall type discussions, and letting the witnesses weigh in with their particular views, so that the members of the community can be better informed about which witnesses to vote for. (Maybe that's what you're talking about here? If so, then I am all-in.)