1. Adding another melee DPS is unlikely, but still possible. Adding ten new Melee DPS is pretty darn unlikely though. People keep acting like we have tons and tons of jobs just waiting in the sidelines when it's likely to slow down pretty fast from here on out, presuming the game even has a ton of time left. It's a pretty old game, and XI stopped getting new jobs around ten years after release, with a number around what we have as well.
2. Stormblood and Shadowbringers both continued to balance the overall numbers across the five roles while avoiding doubling up on jobs added in the last expansion (Healer, Tank, RDPS -> MDPS, CDPS -> Tank, RDPS respectively for the expansions, as you can see no role appears in one and then the one right after), so I'm not sure what the point you're trying to make there is.
3. White Mage and Black Mage are Staff and Rod. Cane is Blue Mage. Otherwise I agree that people put a bit too much stock in how "different" a weapon has to be. We have a surplus of swords, multiple sticks, and realistically any weapon we could get at this point is most likely at least somewhat similar to an existing one (barring perhaps a DoM weapon, which has the advantage of not having to be a weapon).
4. That said, I don't see a gun and sword job being likely at all. You'd have better luck hoping for a Lightning from XIII inspired MDPS than a "gun/sword Assassin" which I feel comfortable going out on a limb and saying it won't happen.
1. Yoshi-P already did introduce a new job. It's called "Astrologian". To say nothing of Scholar, Summoner, Gunbreaker, likely others that are basically made almost whole cloth for this game.
2. No one is saying it's an absolute requirement that it previously existed in an FF. But when there are numerous jobs people want to see then it's certainly going to be important to keep that in mind. Most FFs add a couple new jobs, and XIV already has done that.
3. You're welcome to keep your hope for it, but it's a forlorn one in the end.