If you've ever checked out /r/OriginalXbox there were some pretty neat hardware mods like LCD screens on the front of the console that display current playing content.
The PS2 can play games off of an SD card with MX4SIO through the memory card slot, through USB, through SMB with a Raspberry Pi, and through an internal HDD. You don't even need a working disc drive or a hard mod.
The PS Vita has a rocking homebrew scene.
The 3DS had neat custom themes and being able to use QR codes to download games is pretty nifty.
There's also SBC Gaming devices like the RG350 and Retroid Pocket 3+ that can play a wide array of systems.
Basically I have a hard time narrowing it down. What about you? Is there one console or device you keep coming back to?
I wish I had a TV with less input lag than the one I have, but I have cracked OG Xbox, PS2 and Wii and maybe I don't really come back to them often, but these were such a huge part of my life back in the day. Nothing beats Splinter Cell Chaos Theory in co-op on Xbox and DBZ: Budokai Tenkaichi 3 is still the best Dragon Ball Z fighting game for me. Good times.
Have you looked into XLinkKai? It's basically remote system link and I think there's a few people who occasionally play Splinter Cell.
Honestly, I had no idea about it. We used to play split screen with my neighbourhood friends :D I will check XLinkKai out, however, thanks!