Hi. There is lots and lots to do yet, but lots has been done already.
I'm not sure what you understand by "this project". Our first goal is to release a demo with one mission that is fully playable. My inspiration is the TIE Fighter demo (a video of it).
Most likely, the demo will contain only some intro text explaining Y-Wing Historial Mission 6, some screen to configure your input method (buttons, axes, etc), and there you go into the flight engine.

We are limiting ourselves to clear, reachable milestones so that we deliver something.
If we set our milestones too far away, probably that would lead to disinterest creeping in and end with the abandonment of the project.

Back on topic, you are asking for a progress percentage. I can tell you what I can think of that is missing from the project to reach that goal:

- Capital ship AI, with turret aiming and, perhaps, turret turning. We need this for frigate Vehemence.
- Mission goal tracking, with both updates for goals achieved and goals failed that lead to a final Mission Complete or Mission Failed message.
- Whole hyperspace sequence for the player ship. Currently the player just appears at the mission start.
- Proper inner cockpit 3D modeling to replace my programmer art. Before that, I can't really enable the feature of looking around, because there is literally no 3D art out of the current point of view.
- Realistic looking model for the Nebulon-B frigate. Alternatively, I could ask in the XWAU forums if it's okay to release the demo with their frigate model. The Nebulon-B frigate is a tricky model full of details, and much more work than creating models for several other ships.
- Sound effects. The original idea was to get them at runtime from the original game files. For this demo, though, I think it could be okay to just include some ripped off the game. It's a technical demo, not a fully realized game, so I hope that would be okay.
- Some kind of pre-mission and post-mission interface to show the briefing text and the debriefing text. (*)
- Space dust. It's those particles that move around the player ship to help giving an impression of the current speed.
- Space objects (**). In X-Wing, the planets that can be seen as a backdrop are objects placed on the 3D map. The test mission uses a "Ring World" space object. I guess it's a planet that has a ring like Saturn, not a ring-shaped planet. :)
- Torpedos for the Y-Wing.
- Collisions between ships.
- Player death.
- Testing, testing, and more testing, both looking for bugs and for balance issues. We need to make sure that the test mission plays as close as possible to the original to keep the same difficulty level and the same resolution process. MajorParts has been helping a lot with this lately.

(*) On the briefing:
It could be just temporary menus in the style of XvT even when in the final game they would be recreated 3D interiors mimicking the 2D interiors from the original X-Wing.
We don't need to have the 3D interiors for the demo, neither a fully animated Dodonna pointing at the presentation with his baton. That sounds like a lot of work for something that is mainly cosmetic.
What would be great is to have the "mission powerpoint" working. I mean, the briefing map with ship icons, highlights and panning.
Problem is that the briefing file format is not well described anywhere that I have found. Obviously, the many mission builders developed during the years have had to figure it out. But nobody seems to have bothered to disclose their findings, so I guess we will need to figure it out all over. (At least, this time we are documenting everything on a repository, so there will not be a need for anyone after us to do this work again)


(**) On space objects:
In the original X-Wing, due to memory limitations, these planets used sprites that were quite small for saving the scarce memory that was needed for other stuff. It was even worse in X-Wing 98 and XvT when they increased the resolution to 4x the original, but the sprites remained the same size.
Instead of feeling like you are dogfighting over a planet, it was more like you could spot a planet if you were lucky enough to look at the right direction by chance.
It would be great to replace those old sprites with something better.
I was thinking on, perhaps, replacing them with 3D objects so we can make them as big on the screen as we want, without wasting extra memory. So the mission could really feel like it's being fought just outside of that ring world.
Obviously, it would be a camera trick and the player cannot actually fly to those planets, but I think we could do something with them during the hyperspace sequences, so you actually see yourself coming towards them or leaving them behind, just in the same way that the rebel feel see Endor and the Death Star approaching really fast when they come out of hyperspace in Return of the Jedi.



So as you can see, there is still lots to do, and it's just accounting for the limited scope of the demo.
But I'd say that most of the hardest stuff seems to be behind already (like the iMuse system, the AI fundamentals, the flight group logic) and once the base systems are more stable, it will be faster to add more content to this.

For example, for making this test mission work, I have had to program the whole combat system, with reactions, shooting weapons, aiming, all the instruments in the cockpit, the ship parts system, the colliders, the AI for avoiding collisions, the flight plan system for the different orders used in the mission...

From that point on, adding, for example, another mission or another ship would be relatively small work because the fundamentals are already there and most of the work will be tweaking numbers and creating 3D models.
I foresee that the game progress pace will speed up as time goes, because the work that will be left to do will be easier and more "parallelizable", that is, can be done by other people at the same time.

Another example: making the Y-Wing cockpit functional has taken me months because I had to create all the instruments and their functionality. However, creating the B-Wing cockpit, or other, would take much less work, because it uses the same instruments, only in different places. Only the 3D modelling and some minor tweaking would be needed to add a B-Wing to the game. While it took me almost 6 months to make an Y-Wing flyable, I could make another rebel ship in one day or two tops if someone gives me the 3D art because they are so the same.

So it is really hard to say what percentage there is left of the demo.
If we manage to keep this pace, I don't think it's unreasonable that we all have had a chance to play the demo by around Christmas. The amount of polish, though, is the question here.