Unlock-All was impossible a few weeks ago but after the recent update, many people have already beaten it. Here is a simple guide to show you guys how to do it with ease.

As most people should know, money is the most important resource in this game. In late game, unlocking countries can go all the way to 10Million (or 100million?) so getting money early and snowball is the key to success.

This guide will break into 2 parts:
1. What countries should I unlock?
2. Major city hubs I prefer in each region

Instead of following the "Goal Countries", you should prioritize in big nations (USA, China, India, Brazil, etc..) first.

Priority 1: Get USA asap. What I find is that a long distance route generates more money than local routes.
Grab: France, Spain, Portugal then USA. (or you can grab UK, Ireland, USA.) I prefer Portugal because it generates more money than Ireland and there's no countries that really matters up north money wise (I'm looking at you Greenland/Iceland).

You can get a ton of money from USA. I'll wait until NY pops out and make a route from NY to Paris. It costs about 2500+ when I set up the route. (So save this amount of money first before you unlock USA).

Priority 2: CHINA! China gives u SO many passengers. Even in late game, all my major hubs are filled with people wanting to go to CHINA! The reason why I don't get China first is because you might easily get gameover because of huge demand and not enough money to build airplanes/airports.
Grab: Poland > Russia > China.
Set up routes from Europe to China. I used:
Berlin > Beijing
Berlin > Guangzhou
Moscow > Beijing

Priority 3: Get the far off regions.
Routes get exponentially expensive as you unlock more countries. So it's better to buy the long-distance routes first. I prefer getting Brazil, DR Congo, South Africa.

How to get Brazil: Grab Algeria> Mail> Gambia> Brazil.
I don't know if Brazil also connects to DR Congo + South Africa.
You can set up routes from Brazilia to NY, to LA, Sao Paolo to NY to LA.

Priority 4: Get the high pop countries.
High population = more cities pop up = more passengers = more $.
Get India, Nigeria, Iran, Turkey, DR Congo etc...

Major hubs I use for each region:

Europe:
Germany: 1 Berlin 2 Frankfurt 3 Munich (or can skip). I connect Frankfurt to Munich only and only upgrade planes when there's Oktoberfest.
France: 1. Paris 2. Marsailles
Spain/Portugal: 1. Madrid
UK region: 1. London 2. Glasgow (or can skip)
Scandinavia: 1. Stockholm
Central Europe: 1. Vienna (or can skip)
Balkans: Can skip/Athens/Istanbul
Poland: 1. Warsaw (Takes cares of all the Batlic nations + Belarus + Poland)
Ukraine: 1. Kiev (Takes care of Ukraine)
Russia: 1. Moscow 2. Rostov (South Russia) 3. Yetkaniburg (Central Russia) 4. Irkutsk (Central Eastern Russia)

Africa:
Morocco: 1. Rabat
Egypt: 1. Cairo 2. Alexandria (so many people wants to go there) . 3. Asyut (South Egypt)
West Africa: 1. Bamako Mail
Central Africa: 1. Abuja (Nigeria or can skip) 2. Kinshasa (DRC) 3. Kinsangani (DRC)
South Africa: 1. Cape Town 2. Johannsuburg 3. Antanarivo (Madacascar or can skip)
East Africa: 1. Addis Ababa (Ethiopia). 2. Nairobi (Kenya)

South America:
Brazil: 1. Brasilla 2. Rio/Sao Paolo 3. Manaus
Northern: 1. Bogota (Columbia) 2. Caracas (Venezuela) or can skip
Western: 1. Lima (Peru) 2. Iquitos (Peru) 3. Arequipa (Peru)
Southern: 1. Buenos Aires (Argentina) 2. Santiago (Chile)
Note: Falkland islands is a "country" in this game. But Antartica is not (thank god :D)

Central America:
Mexico: 1. Mexico City 2. Northern City (Monterrey/Torreon) 3. SE City Merida
Islands: 1. Port-Au-Prince (or any other islands) 2. Havana (or can skip)

North America:
USA: 1. NY. 2. Atlanta. (South East) 3. Houston (South) (or Dallas, but Houston always pops out first) 4. L.A (South West) 5. Chicago (Midwest) 6. Denver (Centre) 7. Seattle (or let Vancouver be hub)
Canada: 1. Toronto (Ottawa pops out first but everyone wants to go to Toronto). 2. Vancouver (or let Seattle be hub)

Asia:
China: 1. Beijing 2. Guangzhou (South) 3. Shanghai (East) 4. Wuhan (Centre) 5. Chongqing (Centre West) 6. Harbin(or any North Eastern city)
Taiwan: 1. Taipei
Japan: 1. Tokyo
South East Asia: 1. Bangkok Thailand 2. Burma Capital (or can skip)
Indonesia: 1. Jakarta 2. Surabaya 3. Makasaar (for all the other minor cities not in main Island) 4. Palembang (or can only use Jakarta)
India: 1. New Delhi 2. Mumbai (West) 3. Chennai (South) 4. Hyperdad (Centre) 5. Kolkata (East) 6. Ahmedabad (North West) or can skip
Iran: 1. Tehran 2. Southern City (I use Shiraz)
Kazakhstan/other stans: 1. Nur-Sultan 2. Almaty/any stan capital
Turkey: 1. Ankara 2. Istanbul (can be used as Balkan hub)
Arabia pennisula: 1. Riyadh (Saudi) 2. Jeddah (Saudi or can skip)

Oceania/Australia:
Australia: 1. Sydney/Canberra
New Zealand: 1. Auckland
Islands/Papua New Guinea: 1. Noumea (New Caledonia)

Anyway, this is my first time making a guide. If you have any questions, please comment. Thank you.