First of all: It looks fantastic! Reading that it's getting primed sounds like a milestone to me. I'm super excited to see how it looks in a consistent color, especially all the angles, bay windows, and details of the towers. It should have a different impact when they're no longer white and in a uniform color.
As for possible missing details, I would say that in the middle, next to the doors, 2nd and 3rd floor, something seems to be missing. At the level of the 2nd, 7th/8th, and 13th tile. Maybe it's the combination of both being so close to another that it looks empty.
Some decorative elements like at the very top, or simply adding some air vents would be great. Or even covering it completely with Cities of Death wall elements like you did it on the 4th floor would be fiting. Can there be too much detail on a Imperator Titan like that? Maybe some different building walls would be a good option to get some variety to the different levels.
There is plenty more detail/building that needs to be done.
The internal magazines, the cloister gardens, in fact all of the upper galleries have to be sheathed and detailed.
And don't forget the two main weapons still need their decorative shielding.
I'm surprised no one else has mentioned the two "doors to nowhere ..." situated above the reactor cooling cores.
These will line up with the maintenance bridges when the titan is in the repair gantry.
Yeah, I've got a lot to do still.
I just need to put some paint on the finished parts to maintain my enthusiasm and sanity.