On the subject of new game card formats, I've done a little bit of digging and found what I believe to be the prime candidate for the tech behind the next gen game cards; single-gate vertical channel (SGVC) 3D NAND. In 2017 Macronix published
a paper on the technology, including results on a test chip they had manufactured. I don't have access to the paper itself, but the abstract is pretty informative:
There are a few important things here. It's got very long data retention (40+ years), is described as "very suitable for read-intensive memory", and achieves high capacities at low cost. The sample chip they manufactured was 128Gb (16GB) on a 16-layer process, and they claim that 1Tb (128GB) chips would be possible on a 48-layer process at low cost. It could presumably scale even further in capacity with additional layers.
That seems like a technology pretty much
perfectly designed for Nintendo. In fact
this article about it stated it was "suitable for read-intensive applications such as game-grade memory". I don't know if this is a quote from Macronix or the author's addition, but it would be surprising if Macronix didn't see this as something they could provide to Nintendo.
In 2019,
this article stated that Macronix planned to introduce SGVC after their ordinary (GAA-based) 3D NAND business is established. That's taken a bit longer than expected, but they started mass production of 48-layer GAA 3D NAND in 2021 and 96-layer in 2022. Which would put the introduction of SGVC 3D NAND at right around the right time to be used for Nintendo's new console.
Incidentally, it appears that Macronix have been using a mix of XtraROM and NAND in Switch game cards all along. Thanks to
@LiC who passed the info along to me. Game cards up to 4GB seem to use XtraROM, while cards 8GB and up use NAND. No specifics on the NAND, but presumably it's tweaked in some way to provide better longevity in a write-once use-case. I don't know whether they're using any 3D NAND, but that's only been available from Macronix recently, so anything prior to last year would have to be planar NAND.
This also explains why development on XtraROM ceased after they hit 32nm in 2014, as they switched to a NAND variant. The 4GB capacity of the largest XtraROM parts is the same as the largest 3DS games (although they claimed 8GB, I don't think any games actually used 8GB cards), so it seems like 4GB is the limit of capacity on 32nm XtraROM.