Homebrew Rules

Character Deaths.

The Revivify, Reincarnate, Ressurection and True Ressurection spells are banned from the Kosmos DnD. Instead, if the group decides to attempt bringing back a PC or NPC after their death, they must participate in a ritual. If they do, the character the is to be revived may attempt another death saving throw. The DC for the saving throw may vary, influenced by the time, the character has already been dead and other circumstances (their chosen god etc.). Each character participating in the ritual can attempt to lower the DC by contributing an item of high value.
If the ritual is successful, the items contributed are consumed, and the character is revived at the space where the ritual was held. If the ritual fails, the items are not consumed but noone can attempt a reviving ritual for the next 48 hours.
Additionally, if one or more characters attempt a ritual, while at least one ritual failed beforehand, the DC for the new ritual increases by 2 for each failed ritual before the current one. Furthermore, the time that has to pass before another ritual can be attempted increases by 48 hours for each failed attempt.

Exhaustion.

The table used for the penalties for levels of Exhaustion was completely overhauled. Instead of the original one, each level of Exhaustion gives the players a -1 to all roles up to a maximum of 9 levels. Reaching level 10 of Exhaustion leads to immediate death.

Help with skill checks.

It is possible for players with a bonus of at least +2 in a skill to aid a player that is attempting a skill check. The player providing the help needs to explain how they will try to aid. Based on this explanation, the DM rules whether or not it is sufficient enough to hand out Advantage for the player attempting the check.

Potions.

Players are allowed to drink or apply potions to a weapon as a bonus action. Applying a potion to another creature requires an action and a creature that is not incapacitated may use its reaction to make a Dexterity saving throw contested by the player's Strength (Athletics) or Dexterity (Sleight of Hand) check.

Spellcasting.

Spell casters may cast more than one leveled spells in one turn. Some combinations might be blocked due to their impact.

Tools.

Once during a Long Rest players may roll with a tool they own. If they don't have proficiency with the chosen tool, the roll is added to a tracker. The DM determines the point on which the player is considered (half-)proficient with the tool. If they already are proficient, rolling and adding the number to a tracker may lead to enhanced products (e.g. more potent potions with the Herbalism kit).