One feature I would like is that the chip is capable of realtime handling of packets so that it can run a PROFINET compliant stack. Currently we use a PROFINET gateway that bridges between PROFINET and serial: the anybus communicator (https://www.anybus.com/technical-support/pages/communicator?ordercode=ABC3013)
But this solution is quite expensive.
I understand that such a feature will come with many requirements such as the ability to implement Isochronous Real-Time (IRT) protocol and Precision Transparent Clock Protocol (PTCP). But this allows you to achieve cycle times as low as 31.25μs!

I am well aware of the application of PROFINET in the industrial field. Unfortunately, we have a TCP/IP based solution, so the entry of PROFINET is not in the plan. Thank you for the good information. I always feel sorry for this part, too.

Hey Alan,
I used the W5100 at a previous job, like 15 years ago. It was fun.

As to what should you build, it's easy: 2.5GBASE-T (or higher) with parallel interface and appropriately sized buffer. Like Cypress' FX3, just without the USB hassle, thus much better. :) 

Wow... 15 years ago, you used it a long time ago.. 
Unfortunately, there are no plans for Giga Ethernet either.
This part is occupied by many other big companies. 
Thank you so much for your opinion.