Some of my Homebrew BX Mongrel Ideas

So as I run a game using BX so race=class. I like it because it keeps humans the most common in play. Not because I believe humans are the destined cosmic master race like some slavecoach but I kinda find when I play dnd5 I feel like a furry convention. I have never seen anyone play a human (don't tell me) but I have heard of people playing humans and I believe these wild stories are true. Halfling was the closest I saw. Of course in the event that in 30 years all humans are furies ignore this post and sorry. Half my games are gritty and inspired by the grime and crime of olden times. So I use historic buildings and weapons and armour lots. It makes my weird alien stuff seem weirder. When my furry buddies in-game (I'm a talking crow) meet a deepone fishfolk on the beach we say hi and ask, "do you know where we can find a tavern". In a human-centric universe, that fish guy would steal your children if it could and pretty sure it is up to no good otherwise we would just be murder hobos when we searched his organs for clues.

So the point of this is not to abuse furries (god bless them) and to get their furry fury onto me but to talk about rules relating to my bx inspired class system. I don't have race=class caps. Go for it.

I have other caps. I allow people to change class and racial class (gasp).

0nce+INT bonus times. I also cap class level using prerequisite only so you could max out in one and change then. And smarter people would end up awesomely better off. I cap all classed at 20max anyway. So you can change your dwarf into a wizard or a thief or a cleric. Changing race requires some magic and a wish and possibly make you unpopular but you can do it you pioneering rebels, go for it.

Class Maximums 
All classes are capped at 20th level
You can go as high in your class as you have attributes in your prime requisite
Gaining increases your attributes is a common quest for power, especially to superhuman amounts

Changing Your Class 
You can change your class once plus a number of extra times per extra INT bonus
You need to meet prerequisite attribute requirements for the new class
Bonus +1+2+3
Negative bonus means you can't change
You can change back to an old class you were prior
A Thief at 3rd level then becomes a fighter that is one change
A wizard at 5th level becomes a fighter for 3 levels then back to being a wizard that is 2 changes

When you change class you don't stop being the old on
-use that level in that class for those abilities and spells
-keep all racial abilities cumulative, its a heretical way to become OP
You can use the best weapons and armour permitted by the best class
Add all levels for equivalent levels as a comparison to a character who has not changed
Your most outstanding class is what you are mostly known for
When your total classes equal 9 you no longer gain a whole HD per lever
-from 10th level, you gain points per level depending on your current class
Hanging out with 1st level adventurers when you were a 12th lv wizard is uncool

A nonhuman and humans can adopt a new class can adopt a "human class"
-these are professional classes anyone can just learn with study and practice or necessity
If you change race class from human to gnome or dwarf to elf, magic or a wish is required
No guarantee your new or old race will accept this and you may be considered a heretic
-races might offer this as a procedure or accept you if earn respect through heroism or marriage
-devils or demons might buy your soul for a wish to change your race, it's a great deal
-many might thing a changed person could be a spy or a creepy fan or mad person
-might suspect you just chose to do this for some selfish mechanistic grab at power or blasphemy
-some ignore these silly rules because being an elf makes you almost immortal
-possible havens out there and places that don't care, village dwarf used to be halfling? So what

Consequences
This deals with some of the objections to race as class in BX
I don't really object anyone being a wizard in my setting
Ietmybx elves and bards use wizard or cleric or druid spells (pick one list)
Wanna be a paladin mix up cleric and fighter
A dwarf wizard will bash you in a pub fight and go berserk with his magic axe
Ancient orc elders used to elves in the past age and are near
-immortal but shun iron still
Beastfolk abhuman druids sound cool
Druid and Evil priest misusing that neutral magic
Templar clerics become monks or knights
Old Viking becomes a priest after years of raiding in retirement
Bored knight gets own frontier estate takes up black magic
Wizard stranded on an island for four years no spells become fighter then goes back when saved
Next time character marries gnome or orc in my campaign they can truly be together
Secret wizards offer magic transformations and polymorphisms
Cleric ambassador lived with the fish folk for several years as one of them

I might start a party off as having 4Lv in a previous class as part of an adventure setup

Wizards look fancy with big hats to stand out plus most common spells and public castings are flashy so people don't cause you of unlawfully casting a spell in secret. If someone attacks you and your spell and wizard gear are obvious you won't get the blame. Wizards in disguise or a warrior in plate casing charm is suspicious and possibly a witch.

Other Ideas
Put living standard paid on the character sheet.
It is illegal for commoners to dress above their station
Poor people don't get let into magic shops
poor = 1cp per day for most poor and even middle class really
rich = 1sp per day live in great comfort with servants and best stuff
royal = 1gp a day for flashiest nobility for best luxury and vice

Familiars need expanding to 20th lv
-consult other planar beings for knowledge

Alignment & Oaths
Adding mutations  to benefits (no protection from apotheosis tables)
1-4 Lv minor mutation
5-8 Lv for greater mutation
9-12 Lv d4 minor and one major
13-16 Apotheosis (say goodby to character probably)
17-20 touch causes permanent minor mutation once per person
-can stack benefits to get extra use per person
-with four lots grant a major mutation once per day by touch (one use per person)

Higher-level benefits 13th Level & above
-bonus +2 vs chosen enemy type and extra dice of critical or spell damage
-ability to sense chosen enemy type
-communing with higher powers or patron spirit weekly
-gain a guardian spirit to aid and give you unwanted advice
-protection from opposite alignment +2 save and AC 1m radius (stackable)
-self resurrection (one use on death powers restore you
-plane shift once per week (handy to come back with a second benefit)
-magical weapon +3 with intelligence and other magical powers -4 to others
-healing potion worth d8 appears every day to you
-summon a horde (once) raise a loyal army for a cause
-summon a powerful entity (once) to turn the tide of your cause or aid you
-at zero HP you don't lose any more HP unless burned or beheaded

d12 Higher level oaths
1 Crusader - wage eternal war vs nation or species or crime or vice until victory
2 Search and destroy - secret enemies like witches, cults, changelings or elves
3 Allegiance and service to one of the great rulers, heroes, nations or churches
4 Allegiance to a planar spirit that sends you secret missions through dreams
5 Poverty - give all you cannot use or carry to church or great cause
6 Missionary - zealously spread the faith setting up a mission
7 Sever all ties - all relations and loved ones and property to be left and forgotten
8 Quest - constantly seek lost relics and items lost by the cause
9 Pilgrimage - visit and protect various holy sites or destroy enemy holy sites 
10 Otherworld has called, seek to meet them on their own home plane
11 Build - create monuments like shrines, statues and temples    
12 Hero -wander and protect the commoners of the faith in need vs enemies