Do we still need combustion in a sustainable future? Human activities, principally through the emissions of greenhouse gases from the combustion of fossil fuels, most notably CO2, have caused what has become the biggest challenge of our generation: global warming. Should combustion therefore disappear considering that, so far, combustion was and still is essential to transport us, fuel our industry, make our electricity, or warm our houses? The urgency of reducing our carbon emissions compels us to consider more sustainable approaches, such as renewable energy technologies, gaining prominence as alternatives to combustion-based energy generation. Nevertheless, the intermittent behavior of renewable energy sources, like solar and wind, leads to fluctuations in the electricity supply. In addition, some industry-related sectors are challenging to electrify towards decarbonization. Hence, even in a fully sustainable net-zero carbon economy, combustion will still have a major role, but not by burning fossil fuels. Indeed, to reach clean and sustained energy generation and help the energy transition, combustion must become more flexible, especially in terms of fuel and operation. The objective of this PhD thesis is thus to assess and analyze, using high-fidelity Large Eddy Simulations (LES), the impact of various fuel and fuel blends, such as hydrogen of syngas, to meet the need for fuel flexibility, and/or unconventional diluted conditions such as water and EGR to meet the need for operational flexibility, on the combustion stability, performance, and emissions, especially for micro Gas Turbine combustors.