Simply put, I am picking up a game that one of my previous DM's ran for about 7 sessions. When I asked him about it he revealed to me that the notes he had for the campaign ended up being exactly everything we had taken notes for. So he had a story idea, but nowhere to take it. 

His story was that we were tasked with freeing our deities and killing God (Father, son, holy spirit God), maybe not in that order. The idea is that God was the bad guy and that our Deities (Mine being Lucifer, the fallen angel) were actually good guys. Once God was dead and our 13 deities were free, they would sort of just fk off to the cosmos and leave us to do what we wanted with our world. 

Safe to say, after having gotten attached to my character, the other characters, the little bit of the world that there was, and where the story and our characters would go with our interests, I was a little let down to find out that it was a dead end adventure that never planned on actually going anywhere. So I convinced him to let me run his game with my intent being a sort of testing ground before I create my own campaign. But i'm left with a problem related to running what will likely be a long run multi part campaign. 

In most stories you go on the adventure, kill the BBEG, get the reward, and the quest ends unless the DM can come up with some side stuff for you to use your rewards on and keep the story going. I plan on killing the BBEG (God - Who I have modified into a different Deity for purposes of the lore that fits with my current story) and releasing the demons that wind up being good not because they were good, but because they were driven insane by being in prison for so long that they seemed good. Demons/Gods/Monsters get released into the world, and now the hero's have a bigger mess to clean up. 

As my hero's take down these Gods, I plan on having them attain some sort of gift of the god that will tie into the group in some way. One of my character's has a pact with a Leviathan as a Warlock, and when the Leviathan is dead I would like her to be able to use some unique stats from this Leviathan to help her as she kills the other Gods with her team. The Leviathan is actually going to be a servant of its respective god, so she has to take it down before she goes after BBEG#3 in Part 2 of the quest. 

I plan on Part 1 being a sort of introductory where they go to lvl up and fight God before releasing their deities, but this is something I need to consider before I start up the campaign and realize that I have no fking clue what I'm doing. 

Question 1 - What should I keep in mind when creating these unique abilities in relation to each of the different Deities?

Since I only have 6 players, I want each of them to get a gift related to their class and play style so that they each get something unique, but leave the rest of the gifts to be things that can be bound to high quality items. A sort of steal the gods gift thing to place high powered relics into the world. Sort of like the knife from prince of Persia if the gift of time was bound into it. Or like the Ghin's from the Magi the Labyrinth of Magic Anime [if anyone watches anime]. 

The big thing is the power balance. Giving a God Tier lvl item to some adventurers takes the chance of putting me in a situation to where shit gets really weird and hard to handle. How do I handle that? Is there something online that I haven't read yet that helps explain how to create this sort of home brew stuff? Is there a specific book I should get and read? Am I just getting to far over my head. 

Question 2 - Is there a way to handle post lvl 20 stuff for my characters? 

I expect my characters to probably beat part 1 before lvl 20. I'm hoping somewhere around lvl 15. Once that is done I can actually get started into the actual campaign, But since they will be needing to fight up to about 10 to 13 other potential deities depending on what they do, it makes sense that they would hit the lvl cap really quick. Is there a way to handle post lvl 20 lvls? I'm thinking about just homebrewing for each of them based on their play style and classes, but If there is already content out there I might as well make it a bit easier for myself. 

I have heard talk of some boons and such but I need to look into it again to see what that entails. Should I likely just stick with boons? 

Any other ideas or pointers that can be dropped in relation to this would be a big help. My first time as a DM so i'm basically just gathering as much information as I can before I start writing how all the parts fit together. Making sure I have all my ducks in a row. 

Thanks for the help guys. 

Ok so your DM ran 7 sessions - I'm guessing the PCs are somewhere around level 3 or 4 right now. How long are you expecting them to take to get to level 15? Are you just milestone levelling them every session or using regular old XP? If it's 1-session 1-milestone, the advancement isn't going to feel like much of a reward. Levelling up and gaining levels is fun, but the harder you have to work for them and the more time it takes the more rewarding those levels feel (although of course, there is a limit and it can become frustrating if levelling is taking way too long).

I would keep killing an actual God as the end point of the whole campaign, but have them free the angels/demons as quest objectives and let them take their boons from freeing those. Some of them could be freed and become enemies, some freed and friends. Vary it up. A campaign that has "free 13 of these guys one after the other" is going to get stale. Variety is important!

Remember that as players ascend in level, they are more and more going to want to fulfil their own objectives: high level players build castles, rule kingdoms, gather henchmen and followers, craft their own magic items, and you can't rely on them to do what you want. At low level (tier 1), players can be expected to fulfil short term goals - rescue the kidnapped children, defeat the orc tribe threatening the village. By tier 3, your players will be capable of taking over towns single handed. The actions of the players should have both short and long term impacts on their world and ideally, whilst you will have large, ongoing storylines as the antagonists go about their business, the players will be having most fun when they're declaring war on an enemy kingdom or building a fleet of ships to sail across the ocean to discover new lands.

Having a BBEG is always important but what happens if they decide to side with him? Really I'd recommend creating a world to explore into which you can plan adventures, rather than a plot to complete. By laying out a plot for them to follow, this can lead to railroading players onto a single track (which they might not want to follow). What if they simply don't want to free the demons? Give them a world to play in an explore and they'll help you to create the experience.

Good point on the world to explore as apposed to a plot to complete as it can cause  Railroading problems. 

My idea for the story is that all 13 of the other Demons/Deities that are imprisoned would be released at the same time. Each one is different and will have a different effect on the world. This will result in them having the choice of which to target next based on what they think to be the bigger problem in reality. 

Ex. One Demon would cause the world to have a 3 dimensional aspect to it. Mountains suddenly are lifted skyward revealing many new tombs and releasing monsters from beneath them. At the same token, a different Demon causes plagues of locusts to follow the eastern winds causing famine. While one Demon makes it difficult to traverse the world as a result of the terrain being drastically altered and causing demons to run around, its hard to feed an army with no food. As a result each demon would cause a different chain of effects that would continue to escalate in different ways till they are all handled. Not all of the Demons will cause problems as some of them will remain passive or not have an immediate impact on the world. Ex, Loki and his 3 children (Snake, Wolf, and Horse) won't actually have any impact, but each child will have their own demon that is easier to handle that will help buff the players and making it easier to handle the demons. These 4 demons (Loki and his 3 children) won't actually become directly active in the story till late game when they initiate a form of Ragnarok on the world (Potential Stage 4). 

Anyway, back on topic. 

Stage 1 will basically be the normal campaign. Explore the world, set up for the big fight, just have a good time. Fighting God will be the end of stage 1. Depending on the lore they find and if they choose to pay attention to the lore will determine if they actually initiate stage 2 or 3. It is very easy for them to win the game at stage 1 by simply choosing to not release the demons and or find a way to be rid of them prior to releasing them. Even if the Demons are released, the world would technically stay in balance, it will just be a bit more chaotic. 

This ends up being the same for the Ragnarok stage. It is very possible they could make that stage easier if they choose to go after Loki and his children individually rather then waiting for them to gain power just because they are not doing anything to the actual world yet. At the same token, choosing to deal with these 4 will also make it easier to deal with the other Demons later on. 

My attempt is to create a story that has multiple paths that can be followed. The campaign can stop at the end of stage 1 or shortly afterwards. It can also extend all the way to the end of stage 3. Depending on how things go they could extend it further. It depends on what they decide to do and how they choose to handle things. 

This is why I'm looking into how to handle later levels. If they hit lvl 20 and are only halfway through the full campaign as their playstyle writes the story, then I would be running out of things to give them. That being said, leaving them maxed out lvl 20 since they will be getting things/effects/powers from the various gods/demons/deities might work out just fine. 

I am planning on using an xp lvl system so I can manage how much they get in order to ensure that it still feels rewarding. 

I think that answered your questions... I think...