SA rural resident, early-retired Mathematics teacher. Very experienced GM/DM for Rolemaster (RM) for 30 years, but a fan of all things RM since 1984. Game systems I have GMed, other than RM include: D&D (Basic, 2e, 4e), Dungeon Saga, WarhammerFRP, Starfinder, Currently (May 2020): GMing D&D4e and Paizo's Starfinder RPG - both in-house, Starfinder via Fantasy Grounds. Played systems include the above list, plus Final Fantasy RPG, Star Wars RPG (FFG), Wild Talents.
I like open-world, long-term campaign play, in which game play is emergent (the PCs and game itself will have a life of their own – one can influence the other in dynamic ways – available adventures come and go, whether the PCs get to them or not) and players will learn what their PCs can and can’t achieve at current levels of ability. That is, if PCs are presently having difficulty, go and do something else, then come back later. Be aware that the nature of this style of game means that time doesn’t stop and start at the beginning and end of adventures, but is moving ever forward. If you cannot stop, say, the summoning of a demon within the time frame required, once the scenario “activates”, but leave because the creatures are too tough to beat, the PCs will then have to come back later and slay the demon itself (if they chose to do so). Good luck! If they decide to ignore the situation, the PCs will be affected in some way by their decision, or lack thereof.