Why isn't there any theme, you ask ?
Well simply because nobody wants to do it... paid or not

Thanks guys! Very good feedback! I will ask the developer. :-)

On the other hand: I would even invest my time to get a Piwigo section at themeforest.net but I guess you wouldn't want some simple user to ask themeforest (or any similar site) to establish a section for your piwigo solution, right?

That's what I thought. The guys is not interested because that would mean to much work if it sells it at the same price, or mean a price much higher.
As I probably already answered elsewhere  you can ask for a quotation for free at PigoLabs, the piwigo association, student associations in Web dev, etc or hire on online platform a cheap dev

I had the same thoughts as the OP when I first started using Piwigo. When I moved from a Wordpress gallery I tested literally every PHP based photo gallery I could find and Piwigo was by far the best. The only thing that was a little bit of a let down was the lack of modern themes.

I have been considering for a while grabbing a Theme from Themeforest and customising it to work with Piwigo, but unfortunately I wouldn't be able to distribute it due to the single site licensing terms. Most of the themes cost around $20 for a single site license but for the ability to be able to redistribute I think you'd have to get the full license which is around $1000.

I'm currently using SimpleNG which is the nicest theme I've come across so far.

i also would like to have more modern themes in piwigo - specially for mobile devices (responsiv Design). I'm also willing pay for it - 50-100 Euros. Maybe a developer should Start a Crowdfunding project for it.
I absolutly can can understand that nobody likes to invest time and money to develope a theme without to be sure he can make a profil. but a crowdfunding project would be a good way to see, people are really willing to pay for it.

Quick note on the theme above
Piwigo has core functionality which will match desired layout support for Gallery section. Blog would not be covered off course.
The only "issue" here are transitions. From my experience with gdThumb, there will need to be some careful selection of image sources, but otherwise it is trivial.
Photo page is simply replaced with Lightbox.
So it is matter of time and people willing to pay for that time.

This is on the surface. There is a bit playing around with the layout, but it is trivial.

Note, such "portfolio" theme would probably limit use of 3rd party plugins as it would be hard to provide streamlined integration of all of them.

Piwigo showcase has plenty of decent implementations using existing themes.
So when you say "modern" theme, what do you really mean? Transitions, responsive design, options, etc.?

ok, lets go over your list

* RD - understandable. now to what min for factor you expect to go. Today, PW supports mobile themes which are phone size and then everything above. so we are looking for two themes - one for phone and one for non-phone devices. Themes need to be of the same style for consistency. there could be one theme which work on both size factors, but user experience could be harder to streamline.
* Touch support is somewhat easy to support with right library
* filters - requires better definition of the "smart"
* admin options - this is theme level. GreyDragon, stripped, and similar themes have variety of options. But one need to be conservative as to how much is too much
* Download options - this is touchy topic as many authors would prefer to lock this option down to limit "free" access to full images. PW has already somewhat robust selection of sizes. Making admin area more intuitive may help to address it better.
* Intelligent navigation -  you would have to elaborate. User navigation or admin level navigation. I admit that coming from Gallery 3 background, PW album/photo management is a bit confusing for me, there may be some improvements in the work, but it would be question for PW team
* Gthumb+ - I have created gdThumb which adds more options, but I personally run into issue that in PW, albums and image thumbs are managed by different templates so it is difficult to have streamlined consolidated mix of two. There is a way around it by not mixing two, but I would hope at some point we can mix and match thumbs in one list.
* Full screen images - this is easy with use of plugins similar to Lightbox - i.e. image view full screen is achieved using partial or full overlay. We can even create theme for Lightbox which would take full screen.
* Shiny, glossy... this is personal preference and could be achieved using your own colors.

Compare http://www.pennytaylor.biz/ and http://www.yvonneschleicher.de/. Both are professional sites, first is based of ZenPhoto, second is PW/stripped+gThumb+
Neil did quick migration from G3 which looks simple but slick - http://neiljacklin.com/piwigo/
GDThumb+ did not cover albums, but gdThumb resolves this issue and now you have "tile" thumbs for both modes - http://pww.dragonsoft.us/

I do understand your point to offer pre-canned solutions for 20$-50$, but for that there simply need to be a market for people to offer money for such work.
Example with WordPress is tough, as WP community is much much larger and people are willing to pay money there. There is enough custom work there so same authors can offer packaged goods for few dozen $ as to demonstrate their work which in turn leads to more custom work and reality is that after you do a first complex theme, creating forks is easy.