A lot of people seem to place a lot of priority on the system having a good basic. I can understand that from a nostalgic or beginner programming view. But personally I would like to see a c (c++) tool-chain for the board. I would not expect it to run natively. This would be more suitable to cross compiling. Most of us will have modern pc's available to do the cross compiling on and can run a proper IDE. The Foenix is not meant as a daily driver :) I think.
I have seen some discussion on this board over this, but it only mentions things like cc65 and how "bad" it is. I was however reminded of a video I watched a few years back:
https://www.youtube.com/watch?v=zBkNBP00wJE
and https://www.youtube.com/watch?v=nLv_INgaLq8
In these videos Jason Turner demonstrated using C++17 for programming the c64 (including demo). It has a GUI that shows c++ with next to it the generated assembly in real-time. In my opinion something like this might make a great match with the foenix. It's really my personal opinion, but assembly is just to much effort and good c code is much more readable and portable and I expect people to want to port to it.
On the Apple IIgs we had APW C from Apple and ORCA/C from the Byte Works. APW C was an ancient K&R C. I can't remember who wrote it, but it was a quick port of some existing compiler for another platform because APDA needed something quick. (similarly the APW assembler was a rebranded ORCA/M) ORCA/C is a proper implementation of C89 (the first modern version of C) that came out two or three years after the launch of the IIGS. It's still sold and recently it's gained some support for parts of C99 added. I haven't used it a lot (back in the 90s I mostly used the ORCA/Pascal and ORCA/Modula-2 compilers) but I've experimented with it some lately. I don't know how good its code generation is but I'm sure it's good enough for most uses. If the code it produced was fast enough fo commercial software on the 2.6 MHz Apple IIGS then that same code ought to be plenty fast on a 14 Mhz machine. Perhaps not for animated graphics or any other particularly performance sensitive stuff but it was fine for desktop applications and utilities back around 1990.
Apple eventually shifted their in-house development to MPW on the Mac, using a cross-compiler to generate 65C816 code. (MPW had both an assembler and a C compiler for producing 65C816 code. I don't remember whether or not MPW Pascal could also generate 65C816 code.) The APW and ORCA tools can be run on macOS, Windows, or Linux using Golden Gate (a commercial product) which emulates the 65C816 and translates GS/OS system calls (and parts of the GS toolbox, e.g., the Text Tools) into POSIX (or win32) system calls.
Two problems with using any of those tools (ORCA, APW, or MPW) to write code for machines other than the IIGS:
They generate OMF files (Apple's object module format) with relocation dictionaries, multiple code/data segments to be loaded into different banks, a segment that determines the amount of memory to allocate for the stack/direct page, etc.
The C compiler (and the other compilers for higher level languages) emit code that depends on GS/OS and the IIGS toolbox: the memory manager, text tools, SANE (Standard Apple Numerics Environment … all the floating point code calls into that I'm sure) etc.
The former isn't a huge problem; writing an OMF loader to load the files would be quite straightforward (the subset of OMF that's allowed in executables is quite small) and some sort of executable format for multi-segment files is a good idea anyway to allow for programs that are larger than 64KB. And for a program that fits into a single segment there's already a tool (the MAKEOBJ tool in ORCA) to turn the relocatable OMF file into a binary that should be loaded at a given address.
The latter is a bigger problem and would make those languages difficult to use for non-GS work without implementations of the necessary toolbox code or making major changes to the compilers' libraries and code generation routines. (All the floating point code would have to be replaced, for example.)