Have a long life like the 3DS.

Took PS2 and DS about 9-10 years to reach 150m. Switch will have to last just as long to do the same. If "Switch Pro" extends the lifecycle instead of being a complete next gen console, its a done deal.

Keep a steady stream of strong software support, no replacement until 2023 at the earliest, more hardware revisions, get the base Switch down to $200 or less.

What makes you think they are not selling enough there? Switch is breaking records in a lot of Europe coutries, and even in Asia it's doing very good.

USA is like >80% of America sales, and it's not like Switch is doing bad outsider of that, in Canada for example it's selling just as good, and shipments data doesn't show bad sales.

DS has sold over 150 million in less markets than Switch.

All the Switch really need is longevity. 

The issue with the entire distribution thing and why nintendo lacks in it compared to sony and MS: xbox and PS is piggybacking on a distribution, support, and marketing network that is pre established via MS and sony's other ventures, so it doesn't take them much to support a new reigion- for MS its almost as simple as just shipping the consoles, sony its only barely harder then that.

For nintendo, establishing a new presence in, say, cuba, requires them to build everything from the ground up. Granted, nintendo is famous enough that it doesn't start from 0 as they have great and widespreading reputation, but still.

Have a fair amount of major releases throughout 2021-2023
Sequels to previous games is fine (and encouraged to big games), but they should definitely attempt to expand their already massive collection of IPS to keep things fresh
Continue working with 3rd parties (Someone already stated Monster Hunter Rise as an example)
Develop more games to expand who would be interested in the Switch, for example a fairly large portion of Animal Crossing Users are female, a demographic that Nintendo didn't have a really big game for
Successor no sooner than 2024

Start branching out into newer markets, like the person above me stated.

Price cut next year, keep the generation going until 2025, maybe introduce and upgrade that gets people to double dip.

It can never be. $99 a decade ago is not the same $99 in 2020s, much more in 2024, after 7 years in the market.

I was waiting for someone to take my comment too seriously. I never said it was a reality they will lower the price to $99. I just said that's what it will take to get to 150. Aka they won't.

Drop the price over time and keep making compelling software. A 2D Super Mario with new art style called Super Mario 4 or Super Mario World 3 would sell a lot of consoles. Keep releasing Pokemon games every other year or so. If the Switch has another 4-5 or so years of solid support it can get there, maybe make a Switch Pro, I know I would get it. Give the updates/support that Animal Crossing gets to other titles like Mario Party. Knocking the Next Zelda and Metroid Prime out of the park wouldn't hurt either. Finally Chrono Trigger and Super Mario RPG 2...get it done! Okay that last one is just wishful thinking.