Sounds fantastic, though a fair amount of work as has been said.
I do wonder if the 5e framework is the best fit; A different rules framework might lend itself to this more easily. That said, I'm still running a BX/3.5E6 mashup that's 80% home brew and 20% recognizable D&D rules and its probably the most fun I've ever had running a game. I think 5e would've been even better than 3.5 as a starting point. So I think you can definitely do it. And it sounds really cool!
If you're ever interested in sharing more of your house rules, that would be awesome. You've piqued my curiosity now.
Sure thing...here's what I have so far. I have more ideas for backgrounds, just trying to establish a template for now.
1) Every player chooses a race and a background from the list below. Class is not used. You level as normal, every time you gain a level you get a new Hit Die (always a d10, although some backgrounds get bonus hit points) and your proficiency bonus increases at 5, 9, 13, 17 like normal. Weapon, armor, skill proficiencies are all in the backgrounds. Ideals, bonds, and flaws you make up to fit your character, or you can borrow some from the PHB.
2) Some of the backgrounds are magical (they have cantrips and/or abilities like Warlock invocations), but no background gives you casting ability. Magic generally works this way:
-Arcana proficiency gives you access to arcane magic, which revolves around sympathy between words and materials. Arcanists are always searching for special reagents which can create more powerful effects, as well as the proper procedures to channel them. To make a fireball wand, for example, might require the heart of a fire drake, the wood of a charred oak, AND a scroll that tells you the magic words to bind the magical energy. Arcane magic is good for powerful elemental effects and manipulation of matter, but it isn’t good for affecting people’s bodies, minds, or changes to a larger area.
-Religion proficiency gives you basic knowledge of the Godrunes, the primordial language spoken by the 4 Creators (The Stormfather, the Sun Queen, the Drowned Earth, and the Mother of Monsters). These runes can grant you power over the flesh and souls of the creatures of the gods, allowing one to mend injury, bind or banish both the undead and the Mother’s demons, as well as bind enchantments to strengthen the will and the flesh of a willing party.
-Nature proficiency gives you knowledge of basic herb and animal lore, as well as simple binding chants that are a combination of sympathy and rune lore. (Basically, alchemists and hedge witches have Nature proficiency to make healing potions, restoration potions, etc.) It also gives you knowledge of the nature spirits that live in the Mirror Lands, the hidden home of the elves and other fey creatures. These spirits are often tricksters, but can give boons of magical totems and fetishes that can grant all sort of strange effects. Shapeshifting, weird teleportation abilities, and illusions are the sort of abilities granted by nature spirits.
3) The game is set on a group of islands. At the start of the game, the party starts on a small ship. Everyone makes 2 characters, and may flip between them during adventures. Your PCs and 4-5 NPCs make up the crew of the ship. Everyone has proficiency with sailing stuff, like tying ropes, knowing how to row a boat, etc.
4) Hit Points: Every time you level, reroll all of your Hit Dice. Maximize 1 die of your choice. Add your Constitution modifier times your level. If this number is higher than your current hit point total, make that your new hit point total.
5) XP is not gained from monsters. You gain 1 XP for every gold piece you spend on something non-essential to your adventuring. If you go to town and spend 1000gp, whether on a donation to the local orphanage or on ale and whores, you gain 1000 XP. If you buy upgrades for the ship or cargo to trade, that also counts. You can make money (and therefore XP) by trading, although it will probably be a little slow. If you buy 5 healing potions, or a new sword, or buy ink to scribe scrolls, that does not count towards XP.
Backgrounds
Swashbuckler
Description: An agile fighter type. Pretty basic, your typical pirate.
Weapon proficiencies: All simple weapons, all martial “Finesse” weapons, shortbows, hand crossbows.
Armor proficiencies: Light, medium, shields.
Saving Throw proficiencies: Dex, Cha
Skill proficiencies: Choose 4 from this list: Acrobatics, Athletics, Deception, Insight, Intimidation, Investigation, Perception, Performance, Persuasion, Sleight of Hand, and Stealth.
You gain the following special abilities:
+2 to all of your main hand attacks when using a one handed weapon.
You may add your Dex modifier to any off-hand attack you make.
You add +1 to your AC if you use a shield.
3 times per short test, you can parry an attack. Spend your reaction and roll a d8. You can add that roll to your AC on an attack that hits you. If you roll a 7-8, you may make one attack against the creature that hit you (if you can reach it).
The first enemy you attack on your turn may not make attacks of opportunity against you.
Your speed increases by 5 feet.
