My humble collection! SOTC

Whatever strikes your fancy. 

I really like that collection!  Variety of brands, two divers, one dress, one field, one chronograph, and one Casio. If you want to fully cover your bases, you could add a Flieger feom a brand like Laco or Hamilton. 
 

You could add a popular GMT complication from Seiko or Mido or Christopher Ward. 
 

Another direction might be some fun color microbrands if you want a big collection, or if you want a nimble collection, you might consider moving into a higher price point with a Longines or Oris. 

How about something in bronze or a GMT? 

What’s your budget?

I’m going to suggest something that I’m cheap at, which is to take a moment to take stock and see what you like most. 

You’ve got some nice pieces there do you like the dressy Orient the most or do you prefer the diver look. Do you like the light coloured dial of the seagull or is that blackedout Seiko 5 more your thing. After you’ve worked that out you could check out some microbrands that cater to your likes. 

Or if you’re like me and not so good at that I see you don’t have a titanium watch, and other than that nice Casio all you pieces are mechanical how about looking at Citizen’s Ecodrive Super Titanium range. Reasonably priced titanium watches that will let you see what you think of this great material. 

World timer, GMT, Annual calendar

What about a casiooak

First...Another watch box bro!! Then go micro😎

Great watch collection. Love Hamilton Scuba and Casio 100 the most. Answering your question I'm with @UnholiestJedi on this one. Collection is complete is when you feel so, but not when it checks the boxes for all the types of watches. I, personally don't like chronographs and don't wear suits. So don't own a dress watch. I have 3 divers and plan to buy more of those. It's super personal.

You've got a great collection! do you need another? Maybe a GMT

That's a great collection with variety to cover all bases.  You could start thinking which pieces you want to upgrade and shoot for the $1000 USD mark for each watch.  Maybe Hamilton Murph 38 for field watch?  Tissot Seastar 1000 for diver?  Depends on what styles you wear the most and focus on those.  Slowly add to each category as you gain insights though daily wear and you'll find the right balance

Some sort of German flieger.

A GMT! 
let me suggest this beauty 

Absolute specmonster available january 15.

This Tissot might make a nice addition to your collection. Jomashop has it for well under $500. 

Here's the link: https://www.jomashop.com/tissot-watch-t099-429-36-038-00.html?gclid=CjwKCAiAv9ucBhBXEiwA6N8nYAnfGx1XP5ftNuBa7V_PKeBAiGEYQ-AyanFjbBINtMjn8SgPe2qUUxoC9g4QAvD_BwE

That is a great collection with a piece for any situation.  If it were me adding a watch, I would get something with a very comfortable bracelet like an H-link, etc.  

A pilots watch maybe?

That is a terrific, tight collection you have!  Perhaps add some color with your next watch?  I love the Traska Summiteer and Beaufort Aerotimers both for quality and aesthetics.  I don't own either but am considering both.

Thanks! my mind is still not clear on what I want to add... I am not a lot into fliegers, however I am seriously thinking on stepping up on the next category: Rado Captain Cook, ML Aikon or Tudor 1926. 

On the colorful micro brand I would need to see anyone interesting! Any suggestion?

GMT sounds about right... Maybe the new Seiko GMT but it is quite similar to the 5KX I already own..

Check out Farer for some fun and interesting designs from a quality micro brand. 

Good question. Maybe around 1k-2K maximum! 

Thanks for the suggestion... I think I have a similar situation on the dress watches: I rarely use my Orient Bambino. But I love it.

Maybe I have to address which scenario I need to cover with it. On titanium watches, I think boulder has some really cool options...

That is great! From $1 - $2k I would consider Nomos.  Nomos has several different aesthetics, from more modern (Club) to classic Bauhaus (Tangente).  Their movements are all made in house and the watches themselves get respect from even the most seasoned collectors.  If Nomos doesn't do it for you, from a style perspective, try another German brand, Damasko.  Damasko, makes the DK32, that is nearly indestructible and unscratchable,  with an in house movement, for under $2K US.  

Nomos Club would be a good addition! 38mm in-house manual wind German brand less than $2K. 

Boldr do make great watches. Another micro in that price range that has some interesting titanium options is Zelos. 
I love titanium and have just arranged a trade of three of my watches for a Vilhelm, which I think is a sister company to Zelos and only does titanium watches. 
Good luck with whatever you choose next.