I have the Rivarossi coaches and they are very nice! I run them behind my 39023 Baden IVh

It is not such a simple exercise for Marklin to produce and market a consist of Orient Express cars.
This is because there were so many route variants operated by CIWL (Orient Express), all of which had different consists.
Some examples are:
The classic Paris-Istanbul
Berlin to Munchen (to link with the Paris-Istanbul)
Orient Express train to Manchuria via Russia, and another to Egypt
Nord express (Paris-Scandinavia)
Golden Arrow (link from London)
The Blue Train (Paris-Marseille)
The Mistral operated by SNCF with some CIWL equipment in the 1960s.

But Marklin can quite easily do representative models of the car types used in the Orient express of the 1930s-60s as Bahnhof fan suggests.

Bahnhof fan's suggestion of the WL sleeper types of the 1930s, such as the Y, and the era II version of the BR18.5 steam engine are perfectly achievable without too much research or expense.
The German E17 (example Marklin 37061) electric engine was intensively used by Orient Express in the 1930s on heavily graded sections, and has not yet been done in eraII by Marklin.
Aside from a sleeper or two, a restaurant (WR) car is the only real necessity.
The CIWL trains were from the 1940s onwards, augmented with odd cars from SNCF, SNCB, NS, DSB, SJ, and FS as timetabling, demand, and logistics permitted.
Many of these cars are already made by Marklin.

Marklin would be well advised to liaise with the museum at Mulhouse near Basle, which has a great collection of the typical CIWL cars.
The museum at Odense, Denmark has a WR type car built in 1943, which is a 1930s design.
These museums may even give Marklin a few paint chips.
The technical assistance of museums, will be accompanied by diffusive marketing capability (almost like a Disney licence) which goes with these products.

In regards to materials, my personal preference if cost effective, is that Marklin revive models with true longevity.
This means they should be made of tin-plate (blech).
They could now use plastic detail enhancements in roofs, bogies, and underframes.
The baggage car of which Bahnhof has spoken, was already made by Marklin in tinplate (Primex 4186).
With acknowledgement to Lokmuseum.de


Tinplate cars of length 26.5mm were made by PMP of France in the 1950s, and one example is shown here.
Regardless of scale length, these cars look correct.


Kimball

We mustn't forget that these coaches are also in use today on the modern day VSOE orient express service, so you can justify a rake of CIWL coaches behind modern day French, Swiss, German, Austrian and Italian electrics. I travelled the route with my wife a few years ago for our 25th wedding anniversary and loved the experience of spending a couple of days on these beautifully restored coaches. The "Lalique" dining car was particularly beautiful.

The route that we followed on our trip was London-Paris-Zurich-Innsbruck-Venice, through the Arlberg and Brenner passes. Spectacular scenery!



By the way the modern train uses the all blue livery on the sleeper and baggage cars and the blue and ivory on the dining and bar cars. On the photo above you can see the latter in the distance, about half way along the train.

On my layout I recreate this photo using a red Re6/6, as unforunately my Ae6/6 is a green one!