Ok, what shall we do with the 1 million Americans that live in Mexico, in case Mexico wants to deport them too?

I think the only ones in question of being deported are the ones here illegally. 

It the Donald really DOES try to build his wall and really DOES try to make Mexico pay for it, it will be an interesting...ahem, Mexican standoff.

For those who haven't heard, Trump's plan for blackmailing....er, incentivizing Mexico into paying for the border wall is to seize monetary remittances sent from Mexican immigrants (legal or not) back to people in Mexico. It is unclear whether such seizures are constitutional or practical, but that's the plan.

If Trump does try to starve Mexico into paying for Trump's Wall (those remittances keep the Mexican economy from tanking), it's possible Mexico might retaliate by seizing American assets in Mexico.

Imagine what happens when the CEOs of GM, Ford, and Fiat-Chrysler meet with Trump to explain to him the Mexican government has seized production from the companies' plants in Mexico, throwing their inventory and supply chains into chaos.

https://en.wikipedia.org/wiki/Hermosillo_Stamping_%26_Assembly

https://en.wikipedia.org/wiki/Ramos_Arizpe_Assembly

http://www.crainsdetroit.com/article/20150917/NEWS/150919844/fiat-chryslers-u-s-car-output-to-go-to-mexico-ram-1500-to-switch

Who blinks first?

If these Americans are of retirement age they are already getting Social Security and if they're 65, they're entitled to Medicare.  I don't know to what extent they're covered for medical if they don't live in the U.S., but their Social Security is paid to them no matter where they reside. 

If they are there illegally Mex should deport them.  SS and medicare they are already getting if the are old enough.

I know you meant this as a joke, but it is something that can happen.
Several years ago Americans who owned property in Mexico, had it ripped from under them and had nothing to show for.
I hope for the ones who live there now don't buy property in case that happened again.

I've heard of no problem of Americans being in Mexico illegally.  Is that actually a problem?

Send them to Canada, they deserve the cold for the way they cut and ran from the US.

But you better be quick about it, I hear the canadians too are building a wall ;-))