I didn't see anyone else mentioning these when I started writing. Apologies if I'm repeating someone else!
Gamers and retro-gamers are distinct but related groups.
Both groups want ultra-low consistent latency as priority, with minimal to none post-processing by default.
For modern console and PC gamers, if your panel will support high(er) refresh rates (60Hz, 75Hz, 120Hz as ideal minimum, 144Hz as recommended, 240Hz as perfect) at standard resolutions (1080p, 1440p and 2160p) please include them.
If having AMD's standard variable refresh rate (FreeSync) is an option, that'd be great.
Both a Display Port and HDMI inputs ideally, but I understand if it's HDMI only.
For retro gamers, if you have to make educated guesses as to input settings or processing (deinterlacing for example) please give us the option to override the defaults. And I guess respect what you're told the input is. Don't assume 240p is 480i, or vice versa. You might think that's a given, but it isn't.
If you're going to include legacy inputs (like component video, composite, VGA) please ensure they're have low consistent latency.
Ideally include support for the (now) sometimes unusual inputs from classic consoles or the ability to add it later via firmware updates or as a stretch goal or whatever.
Or at least test the majors from PAL and NTSC regions: NES, SNES, N64, GC, Wii. Master System, Genesis, Saturn, Dreamcast. Playstation, 2, and 3. (Personal request for Atari Jaguar!)
But scalers like the OSSC and RetroTINK line exist for people who need to hook up weird stuff, so there's no massive need to bake it into the TV. So long as the HDMI inputs are low and consistent latency, anything else can be worked around outside.
If including legacy inputs is something you're thinking of doing, please have a quick word with Bob over on RetroRGB and anyone else he'd recommend speaking to.
[EDIT: Also give the option to display as much info as possible. Resolution, refresh rate, connection standard, audio codec, errors, HDCP standard and status etc.]
You may find yourself the primary supplier to the whole retro niche!
And if you become the primary TV makers for gamers, you will never run out of market or free advertising.
If you'll excuse an odd and personal somewhat scope-creep-ish request... If the colour data from the screen edges were accessible, so Hue Ambilight-style tech is implementable without having to point a camera back at the screen or intercept the HDMI stream inbound (as the Hue Play HDMI Sync Box does), that'd be awesome. No idea how feasible in terms of connectivity and dev time that'd be though.