another idea for a Spectrum related PC program has come to my mind:
Imagine a PC program (written in C using SDL2), that creates a Spectrum-like environment. So a window appears that looks quite like an emulator window.
But it's not an emulator, it doesn't emulate the Z80 processor for example. Nevertheless, you can enter BASIC commands in the way you're used from the Spectrum (no keyword-modes though, so more +2 style), and these BASIC commands are executed in the same window on the PC. So this program also has to be a Sinclair BASIC interpreter, maybe it could use the extended dialect of ZX BASIC.
System related commands such as POKEing UDGs won't make much sense, so there should be different commands to achieve these things.
As a result, you finally would be able to write Sinclair BASIC programs, that run fast. Though not on the original hardware (as this isn't possible, except on a Spectrum Next), but in a window on a PC.
There's also no memory limitation as long as your PC has enough Gigabytes left.
There's also no reason, why you shouldn't have pixel-exact movement in BASIC.
There should also be a way to set the program's global speed. In the window's menu, but maybe even as a BASIC command. Or related to the Spectrum's speed, like in emulators: "Run at 600%" for example, but without risk of overstressing a processor emulation. The interpreter environment should stay calm then, while the executed part would run at increased speed.
Actually, I've already written something in that direction.
My script "vintageinput.py" (download here) opens such a window, and you can write something into it (I'm attaching a screenshot below). It's just not a BASIC interpreter (yet), so unfortunately nothing happens, when you enter something.
The script is written in Python/Pygame. The program I imagine should be in C and use SDL2 directly for additional speed.
