Some core features
These are thoughts on high level functions, I don't have enough knowledge to comment on timing etc and the realities of signalling etc may mean you really have to have an integrated LVDS connector, I don't know.
For me even a learning platform needs to enable me to get to a substantial application. My end goal would be to be able to implement a softcore, such as RISC-V but more likely MIPS initially so I can exploit much of the learning material on digital/computer design and maybe implement some retro machines like the Beeb or a Digico M16.
In terms of scale then I might view an entry system (My*) as supporting a RISC-V MPU comparable to an Arduino or mbed ARM, while a more extensive board (BlackEdge) might be a platform for a Linux capable softcore
So a key feature is memory directly connected to the FPGA with the performance necessary for a viable MPU or 6502 or 32-bit CPU. I suspect this means SRAM for the MPU scale and SDRAM for CPU scale but I'm just guessing.
I think it has already been pointed out that the DE10-Nano RAM is connected to the ARM HPS rather than the FPGA which I feel limits how far you can take that platform. The BlackIce however, like the Papilio Duo, has RAM directly connected to the FPGA which for me is a key USP for that platform.
PMODs are essential for prototyping and learning but may be less optimal for application.
Diagnostics and debugging - For me the most helpful role of the integrated (ARM) MPU is to simplify bitstream handling, as BlackIce does really well, and support diagnostics and debugging. I don't want to use it for anything else, I want it to be a stable and robust means to access the FPGA easily. If I need an application processor then I would look to attach an Arduino, mbed ARM or Raspberry Pi or even a second My*/BlackEdge running a softcore system ...
I'd like to emphasis how much I like the approach to bitstream uploading on BlackIce - I only need a simple terminal program instead of proprietary hardware/software that I might only be able to get for Windows. As I gain experience I might use a CL tool via Make or whatever but I can always fall back on the simple serial terminal.