I think your best bet is a Clevo/Sager/Alienware/etc that has a standard MXM slot in it. I think event he recent Asus ones use standard MXM slots now.
Basically, if you get a somewhat larger laptop with fewer soldered parts, your upgrade options are pretty decent. Here's my list of upgrades that could possibly happen in a laptop, ranked in rough order from standardized and unsoldered to 100% proprietary and soldered:
RAM (if not soldered)
WiFi (if not soldered)
ODD (if any)
HDD/SSD (if not soldered. SATA and mSATA are easiest to find, NGFF is hardest)
CPU (if unsoldered, locked to socket)
Screen (if unfused)
GPU (if alternate OEM parts exist OR if it's a standard MXM module)
Motherboard (locked to chassis)
In general making parts interchangeable is what gives you a more future-proof laptop, and unfortunately also makes the laptop physically bigger.
tl;dr: If you want a gaming laptop now to keep up with the new kids later, you'll want one with a standard MXM slot. That's as good as you can expect to do. If you are lucky, Clevo will release a second or third motherboard under the same chassis, and you can change generations of CPUs, but don't count on it. If size really doesn't matter there are laptops that can accept desktop CPUs up to about 95W--those CPU sockets tend to last a little longer than laptop sockets, so you might squeeze an extra CPU upgrade out of it.
BONUS "GATHER ROUND, KIDS" RAMBLING:
I had an Asus G50vt (mid-high end laptop of 2008) and the only thing that's stock on it is the lid, motherboard, chassis, ODD, and keyboard. I could go on for hours about all the crazy things I've done to that machine and how it's still kicking.
The somewhat "standardized" upgrades were the RAM (DDR2 SODIMM), hard drives (9mm and 12.5mm bays, SATA II), CPU (Socket P C2D Penryn), WiFi (mPCIe), and screen (single channel LVDS). Everything else is/was Asus proprietary: The GPU is an electrically reversed MXM module, the OLED screen is proprietary but apparently connects via USB, the screen hinges are nonstandard and I couldn't find new ones so I duct taped my screen in place, the cooling fan and heatsink are pulled from a later Asus laptop actually (G51vx), and the battery I was able to disassemble and refill to 3x the capacity (18 cell battery on a gaming laptop WOOOO EIGHT HOURS OF BATTERY LIFE!!*). But for the most part, if I wanted to I could still upgrade everything now except the GPU and CPU.
It's really not that obsolete in terms of performance, it's obsolete in terms of performance per watt. And that's why it became worth it for me to upgrade to a Zenbook, which has an equivalently powerful CPU, 1/4 the GPU horsepower, but at 1/3 the weight and 3x the battery life. You have to keep in mind that in 5 years, the average notebook power consumption will drop a little more (albeit not as drastically as from 2008 to now), resulting in more of those thin-and-lights and tablets.
So that's my 2c.
*after a restart when the battery capacity meter hits 0 because it measures power used instead of voltage remaining