It should have been geniekin next but i’m struggling with that so i picked up the Pathfinder ARG and converted some races from that. As before feel free to make simple changes.*
Ability Scores: +2 Dex, +1 Int
Speed: 30ft (small, but nippy)
Darkvision: can see 60 ft in dark.
Skilled: Proficient in perception or stealth or animal handling. Proficient in all types of crossbows, and if they exist one type of firearms.
Tinker : Proficient in tools:alchemist kit or poisoner’s kit.
Swarming: Ratfolk have advantage on attacks if at least one ally is within 5ft of target and not incapacitated.
Languages: Common and one of dwarven, goblin or undercommon.
Alternatively try these sub-races that replace the +1 Int and the choice of proficiency.
Grey : +1 Wis, proficient in perception.
Black : +1 Con, proficient in stealth.
White: +1 Int, proficient in arcana.
Brown: +1 Int, proficient in animal handling.
* for a lot of these i’ve turned racial +s to proficiencies. it’s more powerful but i don’t want random +s as 5e doesn’t tend to use them.