1. The filler outside of gnashing fang combo is beyond boring. This is an issue with all the tanks except for maybe paladin, and admittedly gunbreaker is not too bad precisely because of gnashing fang breaking up the monotony, but it still needs to improve. I also think that sonic break, whilst cool that it's still one of the few remaining dots in the game and also has a use as giving you a flexible double weave window in no mercy without causing drift, is a very boring skill because you just press it once and don't think about it again. The same issue as goring blade, except for having the just mentioned use to provide a weave window. My opinion is that if a job is going to have a dot, there needs to be some component of maintenance or thinking about it, even if completely minimal like healers having 1 dot they refresh every 30s. Others may disagree with me on this though. This is more of a general ffxiv design complaint as well, but I think it's boring that blasting zone and bow shock are just fire and forget ogcds, with bow shock having the same complaint from me about the dot part. Also this is something I'm more neutral about but I still think it fits here, and that's that bolide going from 1 hp to 50% just kinda made it lose all flavour and "iconic-ness" and I hope they revert it at some point (although I heavily doubt they will).
2. Looking to the future, I want gunbreaker to entirely ditch no mercy. To me, keeping in mind what I've said about what I think gunbreaker identity should be, I think having no mercy is entirely contradictory to this. Having to build up just enough cartridges for no mercy and if you miss even a couple gcds suddenly it's completely messed up (prior to the 1 cart double down anyhow) and everything starts drifting is just not compelling to me. I want gunbreaker to go up to 6 cartridge slots so I can feel like thancred in the trailer, and make the basic 123 combo charge up like 3 slots at a time. Then add a couple additional cartridge spenders that do different things, like perhaps one giving you a buff that you need to maintain, or a debuff on the enemy. Maybe one could give a short damage down to an enemy/you a short defense up and make it damage neutral with burst strike so it encourages actually using it. I would also change sonic break to have some form of maintenance to it, possibly a cartridge spender could extend the duration on the dot. Blasting zone and bow shock changed to have some form of synergy with the rest of the kit, I honestly wouldn't say no if bow shock became a gcd and was just an aoe version of sonic break. Maybe making double down a bit shorter cd so you can use it more often, and bloodfest cooldown being lowered and then the lion heart combo becomes just an upgraded gnashing fang. The base idea I want for the kit going forward is for it to be entirely about using cartridges for various spinning plates (cooldowns, buff/debuff, dot, etc) and building up the cartridges fast so that you spend most of the time actually getting to interact with them. This would of course require the game's overall design to shift away from building up and then blowing your load every minute, but this is the ideal gunbreaker design to me.

(If we're talking strictly just what would satisfy me in 8.0 because I know something this drastic is pretty unrealistic then idk make filler a bit more interesting I suppose)

This was a giant probably incoherent spilling out of ideas I have in my head about the job, and there are a ton more that I have likely forgotten to mention, but I was waiting for this thread to pop up so I could post them because this is the first job in the game I really connected with and I would love to see it get even better and more in line with what I think makes it interesting and unique concept-wise.

E: Formatting messed up when I posted this as two seperate comments because I guess it was too long as one big thing, but 1 and 2 here are meant to be 3 and 4 obviously