Now it looks like that library will be expanding as Nintendo has since announced the “Expansion Pack” tier for Switch Online. This will be a new subscription tier for the online service. Pricing is currently unknown but Nintendo said that more details will be revealed later.
However, one of the features of this tier is that it will give gamers access to a library of Nintendo 64 and Sega Genesis titles. This will be in addition to what Switch Online subscribers can already access, so presumably this Expansion Pack tier will be priced higher than a regular Switch Online subscription, but there might be a reason to choose it over the regular version.
This is because it will grant players access to games like Mario Kart 64, which will now be playable online with up to four players. This new Expansion Pack tier will be going live in October, so we should have more details on pricing then.