If you're into FPGA programming, you should give the MisterFPGA a try. It's a compact evaluation board for a reasonable price (in relation, what you get for it). There are a large number of cores available - open source. There is even a Atari 2600 Core.
I can understand your interest in adding better video, RAM, ROM and other things to the system. But… as Keatah wrote, the charm of the 2600 lies in it's simple, minimized design. If you wanted to program it, you had to race the beam. This is one of the most interesting real time programming environment, I've ever seen.
Anyway, I am wondering, if it's possible to design a new board with PAL & NTSC capabilities. This either means, you have to have both versions of the TIA built in it or replace it with an FPGA. Examining the boards of the JR version as an example shows, that these are the same for PAL and for NTSC. Slightly different assembly in the video part and different timing components.
I personally use mine with a CRT display, since a lot of games don't have the same touch on a TFT display.
My humble opinion on it.