Howdy, y'all

I might could be DMing a campaign set in a homebrew world I've been working on for a while now. Imagine a Huckleberry Finn riverboat adventure set on the River Styx. It's a southern gothic domain of dread ruled by two families stuck in a blood feud the Martins and the Coys style. Below are some aspects of the world I've been trying to figure out and any help or feedback y'all could provide would be mighty appreciated. 

Firearms and other weapons: Guns are tricky to use in Dnd but they are a huge staple of the South. Would it work to reflavor bows and crossbows as different types of firearms, and completely erase other weapons like swords and maces? You could wield lumberjack axes or hammers like John Henry if you really want a big melee weapon and short swords could be reflavored as bowie knives, but most everything else doesn't really fit the setting. I know this is problematic for certain classes that rely on melee weapons but I'm not against limiting character creation. Is it too crazy to allow Barbarians to use ranged weapons like shotguns and still rage?

Spells: I'm going for a Southern Gothic/Appalachian Horror feel and one of my players suggested limiting the spell list to create more setting appropriate characters. This isn't a sword and sorcery or high fantasy setting so I've been thinking about doing away with the Evocation spell type. It would drastically change a lot of spell caster builds and I think it fits the more witchy, folk magic feel I'm going for. Is this a good idea or too crazy? 

It's a good, crazy idea. If you and your players all buy into it , that'll be part of the fun!

Sometimes, it's fun to work without something considered core to the game. My current campaign lacks deities of any form. Ever. So, there is no cleric, paladin, etc. option. It's not for everyone, but it works for our story.

There are guns in the new PHB, and in the old DMG, so you could look at those. 
For weapons, I’d say take a liberal approach at defining and reskinning them. I agree lots of things might not work, but you could easily call a sledgehammer a maul, it might be reasonable to call some larger knives a short sword. Clubs and staffs should be plentiful. And don’t forget farm implements, a pitchfork or a scythe could be a reskinned spear or glaive (I know that’s not how a scythe works, but maybe this person tinkered with something). Really it’s armor that’s the tougher one, imo. Leather and padded, even hide is easy, but chain mail or plate, now that’s tough.

For spells, you could go more steampunk, flavor wizard spells like tech and throwing grenades, similar to the ideas for flavoring artificer spells. 

I’d shy away from a barbarian with a ranged weapon, personally, but I doubt it would be too game breaking if you allowed it.

Maybe just in general tell your players the ideas and tell them to invent an appropriate version.

I've run a fair number of highly homebrew campaigns. Much of what your talking about sounds like it will make for an amazing campaign for your group.  Here are some thoughts on your specific questions.  Generally reskinning or reskinning and tweaking is always preferable to total homebrew if your players use D&D Beyond. 

Bows reskinned as guns totally easy to do - Short Bow - Revolver / Longbow - Rifle / Crossbow slight tweak to shorten up the range and give it a small aoe and make it a shotgun. 

I would keep mechanics pretty stock if a player wants to play a range class use the mechanics from ranger - it just keeps the mechanics and skills simpler and easier to manage. 

As for magic - I think it would be a mistake to take out an entire school - instead limit the class choice to half casters or melee this caps the spells at 5th level and below and you can call the classes whatever you want but at least this way they have access to some of the basics - cantrips - light - mending - some healing -  it would be  "low magic" campaign by definition as 1/2 casters get less slots and no access to the really big stuff - wish - mass heal etc 

Although to be fair so much of D&D is built around magic even the 1/2 caster thing may not be a great solution depending on if you are looking to run the campaign all the way to 20 - you would then need to de-magic some of the monsters so to speak or the group would be getting wiped out all the time. 

Best of luck sounds like a great concept! 

This sounds really fun! 

I would agree, don't remove an entire school of magic. Especially one of the most popular ones. Limiting to half-casters or multi-class only for casting classes gives a "you only discover magic by taking this specific journey" feel which might be perfect for this.

You're already reskinning your equipment, you could easily do this with your magic too. Instead of a mystical magic power, your casters can be alchemists or tinkerers and their magic abilities can be steampunk-y technology.

Pitch your ideas to your players and see what they come up with too!

Please let me know how it goes, I have had similar ideas and want to see how it can go. Also I agree don't remove all evocation spells just flavor them differently to fit your campaign. Maybe have them act similar to Artificers where they have artisan's tools as a spellcasting focus.