10m subscribers isn't unreasonable given how important online is to Minecraft, Fifa, Mario Kart, Splatoon and Smash and the fact it's portable online too. (PSN 21m June '16, XBL 48m Jan '16)

That's half a billion dollars at $50/yr.

Assuming Nintendo doesn't just give it all to shareholders and decides to use the cash for our benefit, where should it go?

DLC - Nintendo could give members DLC for Smash, Kart, Splatoon and any future online games free of charge.

Price cut - They could drop the price of the console to boost sales/userbase in the hope that 3rd parties might take notice.

Moneyhat - $500,000,000 could go a long way in persuading 3rd parties to port games to the system or steal exclusivity.

Other - Keep servers up for 20 years? Host real world tournaments? Make 10 Mario Kart players a week a millionaire?

Assuming they only keep 100 billion, I think it'd be smart to use some of it to improve the online service and to keep increasing the greatness of the switch.

They will probably just keep it in the bank...

Out of these...price cuts. But ideally, some should go to improving their online because I know that they're aren't going to hit it out the park.

They need to improve infrastructure first and use a portion to maintain and continue improving. Other than that it would be nice to use it for price cuts and deals.

You really dont know who has more money between Nintendo and Microsoft?

Nintendo has alot of money, but nowhere near the amount Microsoft has.

First and foremost, dedicated servers for Smash/other games that demand low ping.

Secondly, funding second/third parties to make installments for series that haven't had a recent release. F-Zero, Earthbound, 2D Metroid, etc. Preferably keep these fairly low budget, so that if they don't explode onto the market it doesn't make Nintendo wary of giving them another shot.

If there's any leftover after that, use it to hire some people with common sense to explain to Nintendo's board why only releasing a couple games per week for the eShop is a really stupid idea.

This is not a good question in my opinion. Obviously Nintendo is going to simply put it in the overall revenue pool, so the real question is what Nintendo should put effort into. The chances are that the online service as a whole needs more work, so that's one place where Nintendo should put effort.

As with any subscription on consoles I think it should be used to strengthen the online network, and build a competitive OS.