Bring back the size and shape of Genesis carts. Not Mega Drive. Genesis had the best size and shape.

Lol. When you put it like that, haha. 👏 

I'd love that but I guarantee new AAA games would be $80 in that case, if not $90. UHD Blu-ray is maybe $2 for the disc and case per unit. Depending on the print run, it could be a little more or a lot less.

I'm struggling to find production costs for 128 GB and up SSDs, but there's little chance it's under $5. 

The future of physical games for PlayStation and Xbox seems to be optical discs and the next consoles will likely only have them as add-ons with no built-in models. 

Nintendo used cartridges during the N64 for faster load times and less piracy, but it cost them dearly. And on the price side...

A relic from the past, Forbes compared the PSX and N64 in 1997, average prices included, $35 for PSX games and $75 for N64 games : r/Games

In all fairness, PS1 had way more titles (some lower budget or even shovelware). Nevertheless, $75 per game on average is not a good look. It's not like all those games were masterpieces or even all great. 

I'd be fine if they brought these back, much like Leynos said:

If modern rigs/consoles could make room for an addon slot for NVME-style cartridges, it'd bring that 80's dream back to life for me (It's what I loved about the save cartridge system in Alien Isolation). 

Well yes, they're getting too large, but it's not exactly a new thing anymore.

As game assets get higher in their quality, so to does the storage needed to accommodate and support those higher quality assets.

Before the console generation even started, I already argued rather heavily it wouldn't amount to much in the way of storage savings.

The issue is that a large portion of game file sizes is due to lossless file types that are not easily compressible, like quality multi-channel audio... The issue is, people would rather listen and fall for marketing.

We don't get a choice. People demand better graphics, then something has to give.

Go back to the Xbox 360/Playstation 3 era and we had drives of around 500GB... Xbox One/Playstation 4 dropped and they came with 500GB drives and eventually moved to 1TB and 2TB.
Playstation 5/Xbox Series X dropped and we are looking at 825GB/1TB and eventually moved to 2TB.

We traded cheaper capacity for faster capacity. - NAND prices are volatile so Sony, Microsoft and Nintendo need to be conservative with storage investment as prices can double fairly rapidly which impact console costs... Which is not the issue we had with mechanical drives.

I have two 16TB mechanical hard drives on my Xbox Series X, I keep all 1000~ or so games installed and updated... As it only takes a few minutes to transfer data from the hard drive to the solid state drive and start playing, where it takes hours to install/download 100GB.
In-fact, I transferred those drives from my Xbox One X, so the data is only additive, just plug and play.

One criticism I have is Microsoft's stupid 16TB per-hard drive cap, original I bought a 20TB drive and even with partitioning it into smaller partitions, the console just wouldn't recognize the drive exists.

ROM doesn't seem to be scaling in capacity much over time.
So Nintendo are starting to use commodity NAND in some carts which is prone to losing data over time, some 3DS/DS carts are already losing data for example.

WiiU's are starting to brick because their internal storage has lost it's data.

There is a slot, it works just like putting a SSD on a PC motherboard. It's under one of the plates, very easy to access.

Needs to be a bit fast and have a heatsink to work at its best.

This has been an issue for years now.

The storage size isnt growing as much as you would think compared to game sizes.

Maybe games should be getting better so you don't 'need' tons of games installed to switch between!

Play one, finish it, then the next. It's been at least a decade since I last did that. Or since I last was consumed by a game so much I didn't want to play anything else at the time.

On the bright side my PC holds 5 M2 drives. I'll upgrade as need be. I will be tossing in another 4 tb later this year, for a total of 8 tb.