I have a couple house rules. Adding fantasy races and attempting to balance them with the classes are a major house rule/hack. Other house rules are listed below.

Encumbrance

Encumbrance becomes more important in these house rules. These rules are expressed in two considerations: How Strength affects the carry capacity of a character and how weight can encumber a character. How often an inventory audit will take place will vary.

Weight is listed in pounds. A “normal” level of miscellaneous equipment (not including armor and weapons) is assumed to weigh 10 pounds. Treasure is added to this, with each coin weighing one hundredth of a pound.

All characters, depending on the weight of the armor and equipment they are carrying, have a base movement rate as noted below. Note that these numbers are adjusted by the Carry Modifier of the character:

Target 20

We'll use Target 20 for attacks. Instead of referring to a matrix, add your Fighting Ability to your attack rolls and if, including the target’s AC, the sum is at least 20 then the attack succeeds. This effectively allows the eating of ascending AC cake while also having descending AC cake (and they're somehow the same cake).

Leveling Up

Hit Dice

Stolen from Knave (or Stars/Worlds Without Number, I suppose): When you level up, instead of simply rolling one hit die, you may roll all of your Hit Dice to determine a new Hit Point Maximum. If the roll is less than the hit points you had previously, simply improve your hp by +1. For example:

A Mage is rolling their HD upon reaching level 2. They had first rolled a 3 for their HP when they created their character. Now they roll 2d4 to determine their new HP. They roll snake eyes, and simply adjust their new HP to 4.

Training

The rules pose the question of training (required in AD&D 1e) to the referee. My answer is training is required to level up. A character with enough XP to level up must either seek a mentor (whose training takes a number of days equal to the new level) or try training alone (lasts 1d4+current level). To progress to 5th and 9th levels, mentorship is required.

If a mentor is available and willing, the character may test the separate attributes for improvement. For each chosen attribute (one of which being a required attribute for the character's class), 3d6 are rolled. If the dice are higher than the current attribute score, the attribute improves by one point.

Agreement with a mentor requires a reaction roll. 3- results in refusal; 4-5 will cost 50gp per day of training; 6-8 costs 20gp per day of training; 9-10 costs 10gp per day of training, and 11+ offers free training. A lasting agreement involving the promise of service, apprenticeship, or guild or order membership will lift each category by one (e.g. 3- means 50gp per day plus the expectation of repeated service while 9-10 means free training with the expectation of services). 

Experience

Monsters

XP from Monsters will follow the following table.