they(UAE) have nothing going on for them beside truism and oil. 0 taxes(with exceptions) brought in a lot of
rich people from around the world who spend their money in UAE which feeds the local economy. there is no chance they would impose
income tax because they would lose these rich people.
there are better places for territorial tax system(georgia,
malaysia, ...) that are not just a desert. and the neighbouring countries have 0% tax as well so they would take themselves out of competition, especially when people can hop right next door.
so i would say both of these are improbable. they already have VAT("sales tax") and they already tax oil companies and banks, the richest businesses. so that is plenty of money to build the infrastructure and run the government. there is not much incentive for additional taxation since there is no need for it.
though, i can see them taxing other types of big businesses, especially in the financial sector.
BUT they might be forced by other countries and organizations to implement at least some kind of income tax. and that i'd say is highly probable, especially of other countries will see capital flight. they will simply pressure UAE thought the threat of imposing various sanctions against them.
even if they would be forced to implement income tax, i am 100% certain they will never implement
capital gains tax on all types of capital because that is how the rich make their money. income tax is for normal people so they don't have to care that much about them.
in the future, when UAE is extremely developed and the land will be more infrastructure than desert, i can see them going Singapore way. but i think that is at least 100 years in the future and who knows how the geopolitical situation will look then.