Yes, you have catched it about nice additions to break the too high regularity.
Balcony is an idea, but i think wooden parts would be better. You said it would be too thin, but no need to make it hollow. An addition of a wooden covered external corridor could be plain, and it would really be a nice addition if you want my opinion.
Like the little part you can see here along the up of the wall... But larger and made of wood :

The stone balcony remain a nice idea, but i think it would be nicer with a very decorated aspect. Not just stones put together. As on your picture. This type of balcony are really architect prowess. They where created only for important castle and clearly where the occasion to decoration, etc, to impress the newcomers.
A small watch tower, thin and high, that you could put on top of a classical tower, or clip on the side, would be a nice addition too. To break the too perfect regularity, again.
Anyway, with those new additions, you're clearly on the good way ! This project if really nicer with them. I will relay it on the french forum.