Adding to your collection is such a personal thing even I don't know sometimes what I will add to mine until I have given it a lot of thought. Clearly you are beyond the stage of having just a few you like to wear or maybe you have not yet made the culling to that level. Buying pesonal items and I include cars, is never buying what you need but buying what you want. So if I were to guess I would suggest the following:
1. The Vacheron Constatin Overseas has tickeled my fancy for quite a while. Classy and in line with the size of your other watches so it does not look out of place.
2. The IWC Ingenieur also falls into this category and similar to the same reasons for #1.
3. The Breitling Bentely GT - new back version looks nice. And if you like the two tone look, see Trusties trimmed in gold. A very nice looking watch that really stands out for the Bentley. I have no two tones but every time I compare all the Bentleys, this one stands out as the best looking. Size is nice too.
That is my suggestions.
Usil