The Aranea was one of the first races I ever did, and I'll tell you, I had a devil of a time making it work. Ultimately, I ended up just going with the presumption that in generic play, any Aranea PC would spend all their time in hybrid form anyway, so I left Spider-Shifter out of the "core" race.
5E Aranea
by VoidWalkerAbility Score Increase: +2 Intelligence, +1 Charisma
Size: Medium
Speed: 30 feet
Darkvision
Arcane Birthright: You know the Prestidigitation cantrip and 1 other cantrip of your choice from the Wizard spell list. Intelligence is your casting ability score when using these cantrips
Spider Climb: You have a Climb speed of 30 feet and do not need to make an ability check to climb difficult surfaces, including upside down underneath ledges or along a ceiling.
Venomous Bite: As an action you may make a melee attack against an adjacent target. On a hit, you deal 1d8 poison damage. This damage increases by 1d8 at levels 5 (2d8), 11 (3d8), and 17 (4d8). You have Advantage when delivering a Venomous Bite to a target that is grappled or restrained.
Fling Webbing: As an action you may make a ranged attack with a range of 30/60ft. On a hit, the target is Restrained until it passes a Strength check (DC 8 + your proficiency modifier + your Constitution modifier). Your webbing can be attacked as an object by someone seeking to cut through it, in which case it has AC 10, HP 5 + your level, vulnerability to fire, and immunity to bludgeoning, poison and psychic. This ability can be used once per short or long rest.
But... I didn't stop. I kept tinkering, and tried to do a version of Spider-Shifter as an optional racial power. As Mystaran fluff demands they be able to swich forms, naturally I've included it here - maybe you guys can help me finally get the race as a whole to work right.
Spider-Shifter: You are a natural shapechanger, capable of switching between three forms; that of a giant spider, that of a humanoid, or your true "hybrid" form. You have only a single humanoid form, chosen at character creation from the list of Human, Elf, Half-Elf, Mul and Dwarf; you cannot assume a different humanoid form. Like a werewolf, your three forms are all you have. Switching between forms requires an action to complete. Assuming the form of a Giant Spider follows the rules for Wild Shaping (see Druid, PHB pg 67), with the exception that you can still talk and cast spells whilst in spider form. In your humanoid form, you lose access to your Spider Climb, Venomous Bite and Fling Webbing traits, but can completely pass as a member of your chosen humanoid race; no Wisdom (Perception) test can unveil your species identity, and magical items relating to your humanoid race will treat you as such. However, in this form, magical items specifically keying off of shapechangers will react to you as such.