Ability score improvements and feats
The Ability score improvement class feature no longer exists. At every even level (2, 4, 6 etc.), you may either increasing one of your ability scores by 1 (up to your maximum), or pick a feat. You may not increase the same ability twice in a row, and you may not have more feats than one-fourth your class level, rounded down. Yes, this means you can pick your first feat at level 4.

Creature types
The following creature types exist:
Animal: natural, nonmagical creatures with INT 3 or less.
Atronach: elemental spirits made of sentient, animated energy.
Construct: mechanical creations programmed to mimic sentience.
Humanoid: ape-like and sentient, no matter the size.
Monster: magical creatures with varied intelligence and anatomy.
Plant: dangerous vegetation, sometimes magical and/or sentient.
Undead: Magically animated bodies or spirits of dead creatures.

Creatures native of Oblivion are Daedric. There are Daedric animals, Daedric humanoids, Daedric plants etc. Daedric creatures have Spell resistance (as the Abjuration wizard class feature), but only against spells cast by non-Daedric beings.

A Ranger may select Humanoid as a favored enemy, but must choose between the following sub-groups: Humans, Elves, Beastfolk, or Giant.
Rangers may also select Daedric creatures as a favored enemy.

External resources
5e.d20srd.org – Basic rules of the 5th edition of Dungeon and Dragons.
elderscrolls.wikia.com – A treasure trove of knowledge about the Elder Scrolls universe. Don’t poke around the historical bits too much – spoilers!

Luck replaces Inspiration
You start the game with an amount of Luck points equal to the average value of your ability modifiers, rounded up. You can spend a Luck point to gain Advantage on a d20 roll. You regain one spent Luck point after a long rest. The GM might grant additional Luck points for brilliant roleplaying, or under particular circumstances.

New skills
Science, commonly linked to Intelligence, denotes a character’s understanding of the laws of the physical world, including architecture, physics, alchemy, mathematics and engineering.
Clerics and Wizards can select Science as one of their class skills.

Gambling, commonly linked to Charisma, denotes a character’s proficiency in games and game theory.
Bards and Rogues can select Gambling as one of their class skills.

No separate spell lists
All spells are available to all spellcasting classes.

Short and long rest
You have an amount of Regeneration dice equal to your CON modifier or your Proficiency modifier, whichever is higher.

You no longer automatically heal all Hit Points after a long rest. You regain all spent Regeneration dice after a long rest. As mentioned above, you regain one spent Luck point after a long rest.
This message was last edited by the GM at 09:30, Tue 13 Apr 2021.

Feats no longer improve your ability scores (e.g. Athlete, Actor etc. still grant additional options, but no ability score increase).

Elemental adept: (Replaces the PHB rule) Choose a damage type among acid, cold, fire, lightning and thunder. You have Advantage on damage rolls with spells that deal damage of the chosen type. You can select this feat several times, each time applying to a different damage type.

Lucky: (Replaces the PHB rule) You have an additional Luck point. You can use one Luck points to impose Disadvantage on an adjacent enemy’s attack roll.

Savage attacker: (Replaces the PHB rule) Grants you the Barbarian’s Reckless attack ability. If you already are a Barbarian with Reckless attack, once per combat when using Reckless attack, you can deal maximum damage with your melee weapon.

Fast metabolism: You have two additional Regeneration dice, but need to eat twice as much as the norm, increasing your Lifestyle expense by 50% or 5 sp per day, whichever is higher.

Spell reservoir (Prerequisite: Ability to cast 2nd level spells) After a short rest, you regain one expended spell slot of a level lower than your maximum.

In order to streamline combat, I've been proceeding as follows, and now would want to make this official.

I use a weighted initiative roll for the opposition, and they all act as a group. Your own initiative roll only determines whether you get to act before the opposition in the very first round. After the first round, you all act at the same time and can post in any order, you don't have to wait for your companions. Of course, you can include conditions, wait for a particular companion's or opponent's action, etc. I will try and keep those things in mind and describe the fight's evolution accordingly.

In combat phases, please try and post once every 24 hours in order to keep things streamlined. PbP combat can really drag on otherwise.