So, I know there have been a lot of speculations about the next agent, so here are my interpretations of Chamber's voice lines:

Chamber to Brimstone: "Monsieur Brimstone, as promised I have found your next recruit. You are off the hook, as they say. Forgive me for doing your job!"

So, this dialogue simply says that the next Agent is coming out soon.

Chamber to Viper: "Viper, your suit is a little stifling, no? When you go meet the new agent, maybe pack something light. Manila is quite warm this time of year"

This one is the biggest indication that the next agent will be a Filipino or from the Philippines. Yeah, this is a no-brainer already.

Chamber to Sage: "Sage, I am told you are something of a radiant mentor. Expect a new student soon. I should warn you that this one is a bit...prickly"

Okay, this one explicitly states that the next agent is another Radiant, but another word clue here is the term "prickly." If we are just going on about personality here, then the next agent is a hot head, but if we are gonna relate this Radiant abilities, prickly might indicate that the ability of the next agent has an irritable or snappish element to it. Since Phoenix already has fire-themed abilities, the only other theme or element I can think of with that kind of temperament is electricity or lightning.

Chamber to Jett: "Zippy Jett needs some competition, I think. I know just the person."

So, in this dialogue, people might have overlooked the term Chamber used to describe Jett. By Zippy he means fast or quick. Indicating that Jett will have a competition over this description means the next agent will pack some speed. And what other element has speed attributed to them? Electricity/Lightning.

Chamber to Killjoy: "Killjoy, I hear your latest project lacks a power source. Do not be embarrassed, I have a solution"

This one is the biggest indication of what the next agent's abilities are. So, we already know she's a Radiant, so Chamber telling Killjoy that he found her a power source means that the next agent has the ability to charge up her projects. The most obvious element that can charge machines or technology is Electricity/Lightning/Energy.

So, the gist is: The Next Agent is a Radiant Filipina Duelist with Electric Abilities. I just figured that she is gonna be a duelist since she might be a hothead and the dialogues indicate that she is aggressive.

What do you guys think?

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EDIT: The next agent is a she because of the voice message Chamber left for Brimstone in his office under the Range. Chamber keeps referring to Agent 19 as a "she" or "her," so this is confirmed.