[Leaving stuff out that is not directly relevant for tor-relays.]
As far as modular, low power, open hardware is concerned, I set my hopes on the new EOMA68 architecture. This is a draft standard for computer cards in the PCMCIA form factor: http://elinux.org/Embedded_Open_Modular_Architecture/EOMA68
Crowdfunding for the first implementation of this specification (EOMA68-A20) has been successful: https://www.crowdsupply.com/eoma68/micro-desktop I ordered one, and I convinced Digitalcourage e.V. to order three more that will be used for trustworthy computing – maybe Tor relays.
The performance of this first implementation will probably be comparable to the CubieTruck which is based on the same SoC, the Allwinner A20, which is an ARM Cortex-A8 design. So CPU-wise, it will be between the Raspberry Pi 2 and 3. Network throughput might be better because it does not use Raspi's peculiar design.
New implementations with more powerful hardware are in preparation (see the latest updates on CrowdSupply), but these will still be smartphone-like systems, not high performance servers.
Last time I checked, Tor did not support the hardware AES acceleration of the A20 SoC called Security System (SS) http://sunxi.montjoie.ovh. Is this still the case?
Would it be a good idea to encourage Tor relay operators to get one of these cheap headless computer cards to run secure non-exit relays that can be left running without consuming a lot of power?
Greetings from Bielefeld, Germany! Christian