lets look at 2 times I ran the same game... one in 2e and one in 3.5e. It was about 10 years apart.Again you prep and you improv. And I don't think it is all that significant if individual things that can exist in a sandbox also can exist in other adventure structures.
Both were HEAVY prep. I had a 15,000 year backstory involving 3 cycles of civilization, and a 'pre that 15,000 years' that was loosely kind noted... the PCs would start only knowing it was 1764 of the common year and 'history' as far as everyone but the elves were concerned went back about 3,000 years... the elven racial memory and akashic monks had data from almost twice that. But the 6 kingdoms that were stated up each had there own take on history (1 of those 6 being the elven commonwealth)... those 6 kingdoms are not at war but also aren't really allied. recently (within last 50 years) a new land was discovered about 3 months away by boat... all 6 want to colonize there. 1 kingdom got a head start, and a second came right up next and between them the formed 3 colonies that all disappeared... and no one knows why. The gods sent warnings about the land being too dangerous. Nobody listened.
game is going to start with you going with the 3rd son of the king of one kingdom, but he isn't taking just his people, he wants to prove that most kingdoms can work together. As such the PCs can choose to be from any of the 5 kingdoms I provide (one is super evil like WW2 bad guys evil)
that is the back story for both attempts... in both I have a planar invasion plot coming no matter what the PCs do (they were starting at 1st level in 2e and 2nd level in 3e so they just didn't have the ability to interact fast enough) and there were 5 artifact swords that could stop the bad guy planar thing coming...
with me so far?
Okay in 2e I had them get attacked at sea twice, then land, and be told they had to scout. Then while scouting no matter what way they went they would run into my invisible tower... this tower held 2 deity avatars that would rope them into additional stories about the scouts of this extra worldly enemy. once they fought the scouts they would come across 1 of the main bad guys... but they can't hurt it... ping no effect. They have to go to the avatars and ask for help... they find out then they had 2 kids a boy and a girl. the Avatars try to fight the 1 main bad guy and 1 of them dies and the other is super hurt but they do kill it... but that shows that even 2 on 1 avatars of the gods can't win... and there are 100's of these things coming. laying hurt on the bed the avatar tells the story of praxton, 15,000 years ago he swore to kill the gods, and the god of forging came down himself and worked with the entire counsel of druids and the grand druid to craft 5 weapons to beat him... but they could not slay him, they locked him here on this side of teh world useing those swords. those swords are the best chance of beating these things, each one would give it's weilder the power to slay even a over deity... she then quested them to find and recover the 5 swords. As they did each one was hidden in a druid built dungeon, each one themed to a magic the gathering land type (forest, mountain, plains, swamp and island) As they pulled each the 'cage' holding praxton weakened and finally at the 3rd sword he was free. But he was too powerful for them... so they had to keep collecting swords. He went to where the evil empire was, and took it over. Then started invading the other 5 nations... all the while the PCs were trying to save the world. Each step planned out, each encounter pre done. Praxton also gets (pre planned) the son of the avatars as a cool death knight he coorupted, and a shadow lich form down in the evil empire. So now he is unstoppable and is going to take over the world and kill all or at least most of teh gods... the last few of witch tell the PCs that they are granting them some bit of divine power to hold, and wait until such time that chosen heroes would rise to over throw praxton and his WW2 bad guy like army... this leads to campagin 2 60 years later. that 60 year jump was planed from the start, praxton getting free scripted, I knew what order my NPCs would send them in at every turn, and if they tryed to say no I had tricks to get them back on the rails,
in 3.5 we started pretty similar (the boat part was pretty on rails) but when a PC said they wanted to counter attack the ship attacking them from that evil 6th country they took it over... and now had 2 ships. When they got to the 'new land' they now had more supplies and the PCs had taken charge. They sent others scouting and they delt with building defenses. NPCs found the tower and the PCs decided to investigate, but when they did I didn't have the avatars tell them they had to do things just warn them of dangers... the PCs controlled were to go and missed the scouts until the enemy attacked there settlement. when they did instead of 'not being able to hurt them' many of the 8 (yes big group) figured out ways to fight the thing... and 1 PC DID call for help from the avatars that didn't die this time because PCs were there and helping... but they still got the story of praxton, but now instead of 5 dungeons to do in order they had there choice of how to handle it. They decided to only go for 1 and leave 4 holding praxton.
they also decided to not wait for an invasion... to go to the planes and try to stop the on coming threat there... but to do so they asked the avatars to teleport them back home where they brought word of the threat, and assembled an inter kingdom force to stop them. By the way the PCs didn't even go, they sent an army of NPCs lead by not just the 2 avatars but had them assemble an angelic host to go with them... and they gave the 1 sword they had to the leader NPC. At this point I have no Praxton, I have no major threat, but one PC points out that evil empire might try to take advantage... so the PCs head south back on the main land to try to make raids and make it look like the goodish empires are at full strength... once there they find out there is this wizard that is trying to mix a ritual to become a shadow lich... and 1 PC thought even if he was SUPER EVIL (N word evil) he wanted to try to study this... so they ended up making friends with him, betraying him killing him and taking the research and start to modify it. They also over threw the evil kingdom, and started a new democracy (first in the world) in it's place... by then the army with the avatars returned to tell the PCs and the kingdoms that they delayed but could not stop the invasion... but that was Okay cause now the PCs had a plan, and they went to the moon (you read that right... and no I had no moon plans) they modified the ritual that the shadow lich was making... but instead of sacrificing people they had the avatars still alive each give a portion of there power to help the (now only 7 PCs one moved away) into qusi liches that could channel the light of the moon.
So then they went to the last stop the invasion would make before hitting the prime material... and i expected this was to stop them. BUT Noo nothing that simple, they came to bargain.
so now I needed to on the fly make up a reason WHY my invasion started and what they wanted other then 'twirl mustach bad guy' and as such the super powered PCs negotiated for them to take the 'new world' land mass as long as they guarded Praxton, and then went and formed a new UN like thing among the now 9 or 10 kingdoms.... (cause somewhere in there I forgot about the sidetrek to the underdark where they talked to the drow, and the organized cult/country of kobolds. also the over throw of a good kingdom to replace it with 2 new socialist things)
I had VERY strong rails in 2e. In 3e I was just starting to make sandboxes (it still had some rails but it was the beginning of my current style all be it crude)