No, the TIE is the perfect example because it sets the boundary on what you feel you're allowed to change. These things are all canon, yet TPTB here have decided that some things will be sacrosanct and others are up for grabs. I don't really have a problem with this, but recognize that it is exactly what happened. I mean, in my version of SW, only ANH is canon and everything else was just conjecture. The other movies were just sequels and prequels and we all know that the sequels and prequels are never as good as the real thing. So in my version, the B-wing is called the T-wing and the A-wing looks like McQuarrie's concept art and not the horrible thing that ILM did to it. See what I did there? Really, though, I prefer to take the approach that it's all canon. If something looks like it needs a re-do (like the A-wing, that one I was actually serious about) then fine, go for it. Offer it as a variant. Look at it like the FW 190 A vs FW 190 D: Clearly the same fighter, but also clearly different versions. Or look at it like the A2 leather jacket (also WW2 era). These things were built on contract from a set of specs. Every factory followed the specs, and jackets from each factory looked different, even on points that were specifically called out in the specs.
Now, on the AG... I can't get behind the argument that the AG "looks too Rebel." If you don't want it with color and rust that's fine, but there's nothing wrong with the shape.* It looks like a combat derivative of the Lambda, which fits the backstory fine.. I would probably opt for Starslayer's cockpit over Fractal's, but otherwise they're both good to go. As was pointed out in some of those other threads (and seemingly not really acknowledged), the X-wing was an Imperial design. It was just stolen and given/sold to the Rebels.
*The fuselage is always going to look like three sticks glued together, unless you put a hood on it like the DeSanto Viper's.
I'm not attacking Rhettro. I applaud him for tackling a difficult design. When you share your work you know there will be some people who like it and some who don't (the response I received to my XJ X-wing was disappointing, but I don't let it get to me because I'm the only one who will have models of it and it's how I want it to look). I expect a similar response here. I'm okay with it if it's the way he wants it and TPTB here sign off on it. I won't use it, but I still respect his integrity. There are some designs that are just blah. I hated the E-wing, in all of its close-but-not-quite-the-same "official" iterations. Then came Starslayer and BrandX and, aside from the side intakes, the E-wing is now awesome. For me, the AG is similar. I've never liked the design until I saw what Fractalsponge did with it. Then I saw Starslayer's. The AG is now awesome. There's wanting to make something your own, and there's recognizing that "as good as it's likely going to get" has probably already been done. (It's like the Camaro. It was perfect in the late '60s/early '70s, and it's been all downhill ever since.)
Fractal's work is interesting in that he may be just a fan, but several (8, IIRC) of his designs are now official (from his work in The Essential Guide to Warfare). Those were all cap-ships. I would not be surprised if he worked on another book and his TIE/Variants and other Imperial attack craft became official. At that point I assume the argument will be back to "yes, but it's not in the first three movies, so we still get to change it." The point being, you can always justify doing whatever you want.