For me my top 3 would be:
- 1) Consolized 3DS with some sort of native controller input/bluetooth capability. After the Switch came out, I realized I don't want to be stuck playing a handheld ever again if possible. On the Nintendo side we already have multiple good solutions for Gameboy through GBA, but nothing currently available for DS or 3DS, and the previous 3DS solutions were focused on capture capability and routed through USB. 3DS Custom Firmware allows for bluetooth controller support, but it's routed through a computer and then through WiFi to the console. Honestly? Some sort of connection with the Wii U Gamepad would be awesome, but highly unlikely- it's a great controller/tablet hybrid that would work well. The touchscreen aspect would be the real make-or-break moment for this project I figure.
2) Sega Saturn permanent save RAM replacement. Internal save RAM was a novel move at the time compared to buying multiple memory cards and juggling them around (where's the community x16 PS1/PS2 memory cards btw?), but the Sega Saturn's RTC eating the RAM battery's life and making you lose all your saves if you don't back them up isn't the sort of thing I'm used to when we're to the point of automatic cloud save backups and near-infinite local storage these days. I've seen some people experiment with connecting large external battery packs, connecting up hacked circuits for rechargeable batteries, disconnecting the RTC from the battery to improve its life, and probably more, but there's definitely no convenient, universal solution short of remembering to back up your saves. I'd love to see a relatively simple adapter/circuit mod that would allow you to have non-volatile flash RAM on the board without having to ever worry about your saves being deleted again.
3) PS2 HDMI output (this one's in-progress!). With homebrew and softmods for the Xbox, Gamecube, and Wii, the PS2 is left as effectively the only 480i console in my setup. Last time I tested GSM there were a large number of games that halved their vertical resolution when 480p was forced, or a portion of the image got cut off due to framebuffer resolution differences. While I'm not a fan of most of the current HDMI mods out there due to their cost usually being well over $200 with installation services (UltraHDMI, DCHDMI, Hi-Def NES), when you could simply get an OSSC for less, get fairly comparable results, and use it for more than just one system, the PS2 really needs a robust de-interlacing solution that's going to end up with you investing a good amount of money into just it anyway. This is the one console where I would be interested in a robust, expensive HDMI solution just for it, because unfortunately it really needs it. Now if only the super slim with the internal PSU would be supported...