Revolution not possible in a democracy

This assumes the democracy actually functions. Taking the cynical view of America, for example, one could argue that even though we are a "democracy", it doesn't function as one. We have leaders taking advantage of the ignorant among us (also supporting policies that keep them ignorant) to retain office while they act against the best interest of their constituents.

So, then the question is: is it still a democracy, and thus part of this discussion? Or is it not a democracy, thus not relevant to your point?

you can revolt against the system with which people get to choose the leader. I believe that democracy is neutral and potentially beneficial but can be easily rigged by lobbies and sbmerged powers. A good cause would be to have a lobby proof voting system.
I believe the answer is liquid democracy given that it would be possible to develop a system that favors decentralization of powers and diffuse democracy on the territory

I think one improvement would be direct voting for the leader of the country, which from what I understand isn't the case. Entrenched safeguards and measures to guarantee transparency at the Federal level, getting rid of lobbies or at least forcing all disclosures to be made public would be nice, and get money out of elections. I think one of Scandinavian countries actually funds elections through public funds, keeping everybody on equal footing.

Regardless of how much the elections in the USA are influenced by money, I don't think people would argue that they are rigged. I.e. people do actually vote for the members of Congress. So if you could convince a majority of people to start a revolution (a lot of work) then you could more easily just convince them to vote for better candidates.