Ability Score: Dexterity +2
Age: Mousemen become adults at the age of 8 years old, and can live about 75 years.
Alignment: Mousemen are mostly good because of their strong sense of community and mutual cooperation.
Size: They small size, between 2 and 2 1/2 feet tall when standing erect on the rear paws.
Speed: Mousemen basic walking speed is 25ft. If they have both hands free, and are not wearing heavy armor, they can scurry on all four paws for a speed of 40ft
Darkvision: They can see in dim light within 60 feet as if it were bright light, and in darkness as if it were dim light. They can't discern color in darkness, only shades of gray.
Nimbleness: Mousemen can move through the space occupied by creatures at least one size larger than them.
Keen Senses: Mousemen are proficient with the perception skill. They have advantage on tests based on smell, and can identify individuals they met before by their scent if they are within 20 feet.
Vermin Resilience: Mousemen have advantage on saving throws against disease, and all effects of disease on them is halved, including damage.
Subrace: you can choose between Heartwood mice, Woodland mice and City mice
Heartwood Mice form villages on the outskirts of rural settlements and are the smallest subrace of Mousemen. They are expert with farming and crafts.
Ability Score: Wisdom +1
Squirm: When wearing light or no armor, they can move through any space a tiny creature can pass.
Artisan Tools: You gain proficiency with 1 artisan tool of your choice.
Woodland mice: living in wild untouched forests, these rugged mice are often distrustful of outsiders, but have learned to rely on other animals.
Ability Score: Constitution +1
Speak with beasts: Woodland mousemen can communicate simple ideas with beasts of small size or smaller (regarding outsiders, predators, dangers, food, water, shelter). Those beasts are also unusually friendly with the woodland mousemen.
Animal Handling: Woodland mousemen have an innate knack with animals. They are proficient in animal handling.
City Mice form their own cities in hard to reach places, or make their own enclave in the sewers or slums of bigger species' towns. They have a chivalrous mindset, and work to the benefit of their community. City mice are often lawful. Flamboyant and sharp-tongued, they often talk themselves out of danger.
Ability Score: Charisma +1
Weapon Proficiencies: All city mice are proficient with rapier, shortsword, dagger and shortbows.
Skill Proficiencies: City mice are proficient with persuasion.
We also applied this ballpoint racial trait breakdown
and came up with this summary:
Darkvision + 0.5
Nimble + 0.5
Subraces All of them: Ability Score: +1 Proficiency: +0.5
Heartwood - Squirm: +0.5
Woodland - Speak with beasts: +0.5
City - Weapon proficiencies: +0.5
This gives the race as a whole a 6 score.