Something simple but useful for every skill. This would likely increase the size of the max guild, or maybe just the garden as each respective 99 is earned. Could be a skilling station or a portal, maybe something to streamline or eliminate an existing daily chore, most of them with optimal banking. I know you said no lists, but here's one anyway:
Agility - Serenity Stone, always using correct stance
Attack - 1 accuracy-related aura reset a week
Construction - Plank Collection for Razmire & Aquarium
Cooking - Range: +20% XP, double produce disabled
Constitution - War's Retreat Bonfire buff, 4 hours
Crafting - Sand Seaweed & Gem Collection for Bert, Arhein & Aquarium
Defence - 1 SoL reset/day
Divination - Universal Divine Location
Dungeoneering - 5 free motherlode shards a week
Farming - Fruit & Flax Collection for Geoffrey, Arhein & Dell Monti
Firemaking - Bonfire
Fishing - Swift Sailfish spot between 2 adjacent tiles
Fletching - 9,000 buyable broad tips/day, overriding stores
Herblore - Potato Cactus Collection for Odd Old Man
Hunter - 3,000 Big Chinchompa points/day, but disables playing it
Invention - Inventor table & Remote access to Machines
Magic - All spells in area are free but give no XP
Mining - Red & Crystal Sandstone from available sources, with robust glass machine
Prayer - Ectofuntus (Potentially pointed west)
Ranged - Next attack will hit for max damage
Runecrafting - Vis Wax Machine with no penalty for error
Smithing - Core metal smithing 10% faster, cannot save or gain bars
Slayer - Task Portal: 50k per use
Strength - 1 damage-related aura reset a week
Summoning - 60 Raw Shards/week, but disables Familiarization
Thieving - Reset all pickpocket cooldowns 3x a day
Woodcutting - Everlasting Evil Elder Tree, no reward, disables Evil Tree
10-Mar-2020 05:17:21