Instead of the avatar of the god of adventuring, how about saying that the god of adventuring was slain by something really nasty? Heroes from all around the world, from all times, and from alternate realities are trying to take up the slain god's mantle. During the game your choices determine the game's ending after you succeed in achieving godhood.
This means you can start out as a badass, or as a weakling with big dreams, and still have a reason to go adventuring. But if you're already the avatar of the god of adventuring ... wouldn't you be out creating adventures for people instead of completing them?
Which would be an awesome game in its own right.
"Hm ... let's see, I'll just steal this lady's locket and toss it in the sewer, grab a few slimes from the bottom of the ocean and stick them in there, and mutate a rat to become a boss monster. Fill a few treasure chests, and we're off!"
You would be rated based on how well you balanced it to the hero, meaning if you give way too much treasure or too little, or make it too easy or too hard, or put in poisonous monsters without adding any antidotes, you'd get a lower score.
Anyway, back to the worldgen idea, you could make it far more likely to achieve certain ends by weighting worldgen parameters before you start. For example, if you want to make sure Elves survive, give them really great defenses but low expansionist values in their culture (otherwise they'd just steamroll everyone else). Or heck, just make all cultures like the Elves a little more so they're more likely to be diplomatic with them.
But I get your idea about going back and forth. I'm toying with the idea of making a time travel D&D campaign. I loved Chrono Trigger, but I realize now that the game was actually rather restrictive and that there were only a few possible changes you could make to the future.
Also I noticed that the only time period that could have meaningful direct impact was 600 AD, upon 1000 AD. Because the other time periods were so far apart, the terrain was generally quite different, and generations passed between them. Just to remind you:
65 million BC
12000 BC
600 AD
1000 AD
2300 AD
End of Time
You might think some interesting things might be seen in 2300 if you did stuff in 1000, but 2300 is a ruined wasteland.
So this heavily limits the number of event interactions you have to deal with.
Of course, there were some long-term impacts during play. For example, one quest had you putting a sun stone in a mountain to recharge in 65 million BC, and retrieving it in 2300 AD. But someone had stolen it in 1000 AD and you need to find him. It's not hard, he has sparkles coming up out of his house. But he's a greedy bastard, so you go back in time to 600 AD and do a selfless thing for his ancestor, which means his family's values change, and by 1000 AD he had been raised to be a generous guy. So he gives it to you
And then you replace the stone in the mountain, and go forward to 2300 to take it.
But as I'm thinking about it, I would need to act like every event in the game is like a room in a dungeon, and they interact with each other in complex ways. If you open the valve in the water wheel room, the floodgates open and the grand entrance opens. Well, if you slay the bandits, help them, scatter them, join them, or take control of them it will have an impact on everything in the area.
We do this all the time in D&D. The problem is that to have a coherent time travel campaign you have to detail out a lot of the events that will happen in the absence of the PCs, and then modify what happens in the future due to PC activities in the past.
Another thing Chrono Trigger did was to say that when you traveled through time, you were really just going to that time period, but you couldn't choose an exact date. As an example that didn't actually happen in the game:
A. 600 AD: you open a chest, taking 100 GP
B. 1000 AD: You go back to 1000 AD and spend the money to buy a key
C. 600 AD: you use it to open a locked chest in 600
But you can't go back to
before you opened that first chest at A and got 100 GP. It's like you're entering a timeline based on some external clock. You can't go back and take the 100 GP you got from the chest a day before you opened it at A. This prevents a lot of paradoxes from popping up. After all, if you went back to before A and took the 100 GP, you wouldn't have found it there in the first place to buy a key with.
Also, it means in Chrono Trigger you never meet past selves. Although it would be technically possible, in the case of Magus meeting himself in the past, although at that point in the story you don't have the adult Maguc in your party.
Also you can leave Robo to plant a forest in 600, retrieve him in 1000, then go back to 600 with Robo in your party. You can stand there with Robo in your group watching a duplicate Robo ploughing. But they don't communicate because you're on the world map.
But that means Chrono Trigger had no real mechanism for preventing paradoxes. It just shoehorned you into the path you needed to follow. It was still a great game, one of the best I've ever played. But it doesn't give many bright ideas for trying to run a D&D campaign.
My players might leave Robo in 600, go to 1000 to retrieve him, then return to 600 and pick him up. All they have to do is return him to his planting by say 700 AD, and they can still fulfill the time travel requirement of leaving him and picking him up again in 1000. But until that happens, they would have two Robos. Example:
600 AD: Leave Robo
(400 years of Robo sitting)
1000 AD: Pick up Robo
Travel to 600 AD: Pick up Robo
Step 3: ??
Adventure.
Travel back to 600 AD right after they picked up Robo: Leave him there
From the perspective of people in 600 AD, Robo was brought in, started work, left for a moment, came back and returned to work. He works for 400 years, then is picked up by the adventurers.
So obviously my campaign is problematic.