The older nodes don't seem obsolete if component reliability is a concern. All my concepts consider their potential. They're quite limited in performance, storage size, and energy, though. There's a tradeoff. Lots of companies want a cheap, reliable, simple CPU/MCU. That's where the oldest nodes shine. That said, the newest nodes are tiny enough that one might make a 2 out of 3 setup with extra error correction like Rockwell-Collins' AAMP7G CPU. Might still be pretty cheap... per unit (not development cost)... on 28nm CMOS or SOI process. Haven't seen an attempt.
Do you have funding to do a MOSIS run or two? I wonder if we could find some.
If you do a project, I suggest 350nm since it's the last node that you can visually inspect the resulting silicon. It's as fast as you go before you need electron microscopes and such. It's also more likely that the open tools for hardware will be able to handle such a node instead of deep sub-micron. Finally, there's old research in transistor-level optimization that might be applied to it in new, open tooling. Might let people do standard cell that inches a bit closer to performance and energy use of custom designs.
