I don't know what your size requirements are, but for a prototype, have you considered using multiple boards?
For instance, you could use three Arduino Mega 2560 boards together.
One could be the "master" - it would have on board the interpreter and other code for your "pseudocode" system; it would use SPI to read the "pseudocode" (and to write variables, arrays, etc) - from either SPI flash RAM chips, or from SD cards - or both.
The other two boards would be used for your display; give over one half of the display to each board. Add an SRAM upgrade to each board as well - for instance:
...there are other similar options as well, along with some other nifty tricks of this nature (google "arduino mega ram expansion" for more).
Control those as "slaves" via SPI as well (ie - hang the SPI flash RAM/SD Card/Display Mega 1 & 2 off the same bus, with different addresses of course).
Those two "Display Mega's" then become a form of a "video card" to control your display. You would code them with a system that can take simpler commands via SPI (ie - clear screen, copy memory, draw a pixel, set a color, etc) and translate them into using the lower level (and faster) display routines for the final output. Since each board would control a portion of the display, they could operate in parallel and potentially gain performance in that manner (and there is nothing to say you couldn't further sub-divide the display, or add additional boards to do other processing, etc).
In short - instead of thinking of the system as absolutely needing a singular monolithic processor that can do it all (which doesn't exist - until you get to a system-on-a-chip or SOC - which, inside, is also doing nothing more than combining a bunch of smaller parallel systems into a single chip) - think of it as a collection of processors communicating and coordinating to achieve the final result.
This is in fact how you design such a system as you are describing. Control and software development, of course, becomes more complex. Plus, the hardware, in prototype form, will also be a mess (most likely). The final design, though, could be shrunk down into something much more reasonable in size, if desired.