This is just a proposal and its open to discussion when there are right arguments like yours. of course i havent considered everything. my main concern was how to be able to play grepo without compromising a normal real life. about revolt worlds i wont make any comments since i am lacking of knowledge, so lets talk about conquest worlds. about sieges maybe we can give the option to a player to break his protection when under siege the same way it happens with beginners protection when you can break it anytime you want by attacking someone. or maybe you wont be able to do anything but the rest of your teammates can attack your cities under siege. fact is that this feature will make conquering a city much more difficult than before since the chances to catch someone off line will be less. as a result we wont see players that have 2-3 times more cities than the rest as it happens today but worlds will be more competitive which will benefit the game. about mechanics if such thing is possible or not i have no clue but i believe that it can happenThere are so many problems with your suggestion tho which you haven't considered.
Revolt worlds: it takes 12 hours for revolt to go red so you have 2 choices, revolt the player right after his 8-hour free window, assuming TTs or around 1 hour for your average revolt, that leaves you 3 hours of RR to land a CS, which is extremely easy to defend for anyone half-competent in any decent team. Option number 2 is to revolt the player in the few hours before his 8 hours so much of BR happens during his sleep time, which again leaves 4/5 hours of the same timeframe to defend. TLDR players on even the hardest frontlines would only need to be playing for 5 hours if being OPed to lose no cities, defending would be so extremely easy that no cities would fall apart from brute force which is pointless (essentially turning every server into bigger alliance wins). And let's not talk about the elephant in the room, 8 hours to build back any walls without having to worry about cats, being able to slow stack any cities without any organization... it's turning the game into baby mode.
Conquest worlds: 8 hours not to be attacked, well simple everyone will just be trying to land CSs in the 4 hours before that, leaving only 4 hours to be able to break a siege. While the issues are not as important as for revolt, turning sieges into 4 hours is insane, plus add to that the fact you wouldn't even need to stack said siege for 8 hours, cities would drop like flies for anyone not hyperactive. In this case, your proposition actually hurts the casual player, because he has to be online every day for the 4 hours before his sleep time or lose his cities for free, but still has to be active the rest of the time for regular sieges. It just seems to defeat the point really...
There are many other issues regarding this point that can be abused which I will refrain to bring up to keep this relatively short, but please in the future consider the impact of your propositions with existing mechanics before you bring them forward