Is there a technology that could theoretically make international flights really cheap?

Define what is really cheap. There are in my opinion a few things to look forward to if you look at the efficiency of aircraft. Firstly aerodynamics. Early aircraft were pure guesswork when it came to aerodynamics, only in the 1980s was there any analytical software that could predict the air resistance with any consistancy. Still, this area of study has undergone a lot of maturing in the last 40 years. Nowadays, CFD programs can comprehensively and with relative good speed calculate an accurate resistance profile for an aircraft. However, only aircraft that were designed after the year 2000 would have profited from this technology. So, there could still be a possibility that newer aircraft could shave off a few percent in air resistance that way. Furthermore both Boeing and Airbus are currently working on innovative solutions to the air resistance problem. Boeing for example is trying to reduce induced drag by implementing a new wingtip design like in the 777x. Airbus is currently working on laminar flow wings, so there might be a few percent of fuel savings there. Furthermore there is the issue of fuel. There is currently a lot of research going on in the use of H2 for Jets and it is something to be exited about if you want to fly emission free. But it is a very inefficient way to produce energy for jet. For one, it will be difficult to burn hydrogen in existing jet engines as the burning temprature of Hydrogen is way higher than the temprature for Kerosine. So the most likely way, that hydrogen will be used is as a fuel cell producing electricity for a propeller. There are also a lot of questions regarding production, storage, safety and the price is very dependent on electricity costs. So it might less costly per Kwh thrust (as hydrogen has a lot more KwH/kg), but that will probably be balanced out by the increased weight of the airplane due to the storage tanks needed. Though in the future, more advanced battery technology will probably make hydrogen moot as an source for fuel, as they will become more efficient per kg, are safer and pose less of a maintenance challenge. But to see that you would probably have to live past 2050. As for personnel costs, there might be a lot coming. There is currently a lot of talk about a one pilot cockpit for large airliners. Though there is a lot of pressure from pilots for this not to happen, as even with four eyes in the cockpit it is a high pressure job. Expect that this technology will mature and be implemented in the medium therm. Unfortunately from a cost perspective, there can't be any savings in regards to the stewards on the plane. There are also maintenance costs to consider and there is a lot of good news on this front. Newer aircraft are getting more cost friendly also on this front. The a220 (or the bombardier C series) for example has made huge strides in this field, and you can expect this to get better for any further aircraft coming into service. A future introduction of hydrogen-electric engines or battery electric engines, will also improve reliability a lot, since these engines operate with almost no moving parts or complex geomerty (as one might see in jet turbine blades). Last but not least, there is the consideration of airline models. Due to the introduction of planes like the a220 and the a320NEO XLR, more and more airlines are currently moving from the good old hub and spoke model to the newer point-to-point model of operations. These innovations in airline strategy will bring cost savings, as they don't have to land their planes on overcrowded and expensive airport/hubs, like La Guardia, Heathrow, ect. For the costumer it is also more time and cost efficient as they don't have to fly the "long way around" to get to their destination.