Other mods have Tie and Xwings, simply just add them.
1) make them expensive
2) create building (hangar) which can spam fighter for planetary defense.
Yeah other mods have them, but they also still tend to fall within one of those two issues that I mentioned- it's very easy for gameplay to became very air-unit centric. Like I said, you either have them be representative of their actual strength, in which case they wreck everything pretty easily, or you try to make them balanced and you really just have an airspeeder that looks like a fighter without doing anything that you'd expect a fighter to be able to do.
One of our big concerns with work on ground in 2.2 was trying to get air units to a point where they feel useful but don't become the powerhouses that they very often were in 2.15, and using ground fighters, you're basically introducing a unit that
has to feel that way to feel correct, and you have to balance around that, in which case you'd basically need it to take multiple pop cap (and which makes it particularly bad for a garrison unit), which isn't always ideal either. Trying to base balance on just stuff like credit cost isn't a great thing to do, because, basically that doesn't make you feel any better when you're actually in the battle and they feel oppressively powerful.
This is why I've said if we're going to reimplement ground fighters, it would be in a different way that's not an actual standard unit.