What’s with the “dead” characters at the end?

I would consider this a symbolic gesture meant to remind the viewer that the outcome of the movie was not possible without their sacrifices. They all wore V's mask and that symbolizes that we can all be V and that what he stood for cannot die because we all stand for it. Good ideas don't die.

source : V for Vendetta Explained

As V says:

Beneath this mask, there is more than flesh. Beneath this mask, there is an idea, Mr Creedy. And idea's are bulletproof!

The book in fact has Evey carrying forward as V after his death, she dawn his mask so that the idea continues on via a new person.

Evey in the movie doesn't do that she instead explains to Finch:

He was Edmond Dantès (V's favorite character). And he was my father. And my mother... my brother... my friend. He was you, and me. He was all of us.

When people start taking off their masks we can see faces of characters new and those who have died. This is representative of the fact that everyone has in some small ways contributed to the change that they have brought about for themselves.

According to the film's co-creator and illustrator David Lloyd, the ending sequence was intended to show that the 'public' (res publica) are of one mind and body, rather than disconnected individuals. By having the people who chose to defy the State (and were killed by the State) shown at the end, the implication is that it's possible for the state to kill a person, but that they can't kill the idea of freedom that those people were willing to die for:

DAVID LLOYD: Well, I think because they stuck so close to the original in the visual aspects of it... all the key instance scenes seemed like they did them in that way and I think quite affectively. I congratulate them the most, if you're asking me sort of about changes, I congratulate them on the final part, on the ending. It was a very clever idea of having all those people in the masks because basically what it kind of symbolizes is an act of mass defiance, which is actually a mass defiance made up of individuals because, of course, V is representing the individuals' action. But the public adopting that persona through the mask and then becoming one... basically it was like all for one and one for all.

It was a very clever, symbolic way of doing everything.

Just throwing this out there for speculation and thought, what if they "ending" is what the people wanted to happen but never did and everyone wearing those masks had actually died from rioting against those troops. The track never started and Parliament was never destroyed. However, what the viewer sees is what everyone who rioted WANTED to see but never got the chance to. It would tie together the dead characters being in the crowd theory.

In the scene where Evey Hammond and Inspector Eric Finch was watching in the distance as the Parliament building was being destroyed, Finch asked Evey who is V. Evey responded that he was you, and me, he was all of us. Now the mask symbolizes V and since he was "all of us" when they took off the mask they were, at that time, V.