Alright, so here are my random thoughts on the subject:
1) The main appeal for me is to be able to play the older 8-bit games on a modern TV, via HDMI, with a perfect picture quality. I want to see the pixels clearly on the screen, with no ugly color bleed or any kind of "wavy" interference which I often see when using the original consoles on old CRT televisions. The systems I'm most interested in are ColecoVision, NES and Game Boy + Game Boy Color (and Game Boy Advance if possible, in the future). The Atari 2600, Sega Master System and Game Gear are interesting too, but only for a few specific games I like to play on those systems.
2) Every time I turn off my ColecoVision I wonder if it will turn on properly the next time. If it doesn't, then I have to send it to Yurkie for repairs, and cross my fingers that said repairs are actually possible. That problem would go away with a newer system with newer electronics, assuming the newer machine reproduces the original machine perfectly (and from what I've seen in your YouTube videos, you've certainly got that covered). I'm also not very interested in purchasing an NES because of its infamous cartridge connector problems, even though there are tons of NES games I enjoy playing. The NES toploader is not an option for me because it only offers crappy RF output.
3) I'm personally torn on the issue of SD versus cartridge port. On one hand, I absolutely want to use my collection of ColecoVision carts on your proposed FPGA system, so if you end up not offering some kind of ColecoVision cart adaptor, that will be a total deal-breaker for me. On the other hand, given that NES carts are getting quite expensive and that I'm not really interested in collecting carts for that system, I would gladly settle for playing NES ROMs off an SD card (again, as long as the games look and sound the same as they do on the original hardware, inaccurate colors/sounds ruin the experience for me). For Game Boy, I could go either way: The carts are small so I could see myself building a small collection of my favorite GB/GBC games on cartridge and playing them on your FPGA system via a cart adaptor, or else I could play those same games from an SD card. I love playing Game Boy games on a big-screen TV (I bought a Game Cube years ago mainly for the Game Boy Player add-on).
4) You should be aware that a good number of the latest homebrew ColecoVision games require Opcode's Super Game Module (which essentially adds RAM and an MSX sound chip to the ColecoVision via the front expansion port). So I can only hope that you will develop a new ColecoVision core that includes SGM support, or else the homebrew games that depend on the SGM will be unplayable on your FPGA system.
5) As stated above, the main draw for me are the 8-bit systems, but I would like to be able to play my favorite 16-bit titles on your FPGA console, such as Super Metroid, Mega Man 7, Rockman & Forte, Mega Man X/X2/X3, Zelda - A Link to the Past, Super Castlevania IV, Demon's Crest, Street Fighter II, Super Mario World, Super Mario Kart, (...and on Genesis...) Castlevania Bloodlines, Aladdin, Golden Axe, Gunstar Heroes, Landstalker, Mega Man - The Wily Wars, Truxton, Wonder Boy in Monster World, and many others I'm forgetting. All this to say that if 16-bit cores are in your plans, I would be interested in them.
6) I'd like to use this opportunity to add my vote for a Vectrex core. It's not one of my favorite classic systems, but it would be really cool to play those games with anti-aliased vectors on a big TV screen.
7) I think there should be two distinct versions of your FPGA system: One HDMI-only (with perhaps some extra audio jacks for those who want to output sound through their big living room sound system) and one with the "older" set of audio/video connectors. The non-HDMI system would be there for those who want it, with the understanding that they would have to pay extra for it. I expect the majority would prefer to have a system designed for HDMI-only, which as you stated would cost less to manufacture.
8 ) One feature I would absolutely LOVE to have in the black-&-white Game Boy core is the ability to freeze the game and set custom colors any time I want, exactly like it was done in the Super Game Boy cartridge adaptor on the Super-NES. The numerical "color password" system of the Super Game Boy was a really neat idea, as it allowed me to reuse my favorite color schemes without having to memorize exactly what colors I selected. In the Game Boy core, it would be nice to have a similar password system, or to be able to somehow save color schemes on an SD card for later use.
With all this said, I would like to request some clarifications:
1) The FPGA can only contain/run one core at a time, right? In that case, how does running ROMs from an SD card work? Does the user need to load a core manually into the FPGA before running a certain game, or is there a system in place for updating the FPGA automatically if I jump from ColecoVision to NES to SMS to Atari on a rainy day?
2) Certain games cannot be finished in one sitting and require a savegame feature (The Legend of Zelda on NES comes to mind) so does your NES/Famicom and Game Boy cores support saving via an SD card?
3) How do controllers work on your FPGA system? I suppose the disc on the Intellivision controller translates to a modern analog controller, while the ColecoVision's keypad extends the need for buttons. How do you plan to deal with those issues?
4) Do you envision cores capable of running arcade ROMs? I would imagine that would be moderately difficult to implement, and it wouldn't be a feature I would be personally interested in, but I'd like to hear your thoughts on the subject.