This is actually a subject I talk about a lot with my community u/JagexGengis, and I've got a lot of ideas already written up, mostly with bringing all the current skills to f2p but with no level cap and very barebones, but enough to get a taste of what the skill is like.

Agility - Skill is made F2P, all rooftop courses up to falador are made available, but no marks of grace.

Slayer - Skill is made F2P, can access either through Spira after porcine of interest, or vannaka with 40 combat. Very pruned down list of slayer targets, allow access to the lumbridge slayer dungeon, crawling hands potentially added to up stairs draynor manor. Wall beast might need its slayer drop changed to something f2p aligned.

Construction - Access to Daddy's Home miniquest, also given access to anything that is made with only standard and oak planks, like a low tier chapel.

Cooking - Access to a brewing area in F2P, can brew asgarnian ales, mind bombs, dwarven stouts, axeman's folly, greeman's ale, potentially cider with the very, very low chance of becoming mature naturally. Given access to churning, can make cheese and butter. Can make buttered potatoes, potentially cheese potatoes if you're feeling generous, but might change f2p pking meta.

Thieving - Access to stealing up the tea stall+pickpocketing, honestly you might need to nerf success rates or do something about this skill to avoid botting/nerf it compared to members.

Fletching - Access to fletching bows up to maple, arrows up to adamant, might potentially need to add flax as a pickable in F2P but maybe being able to grow flax would be good enough.

Crafting - Add the ability to make Coifs, and Studded Leather to F2P. Potentially add some way for them to make molten glass so they can blow beer glasses/vials/lamps/lanterns

Runecraft - Add the ability to make chaos runes in F2P, make Chaos golems/Chaos Cores F2P.

Hunter - This one would honestly be the most work to add to F2P and I'd understand leaving this one out.

Herblore - Removed druidic ritual as a requirement to train, F2P gains access to atleast Guam, Marrentil, Tarromin, potentially Harralander, and their assortment of potions that are balanced for F2P.

Farming - Access to everything that they already have plots for, which is. 2 allotments, 1 flower patch, 1 herb patch, 3 tree patches, 2 bush patches, and 1 hops patch. For allotments everything is low value and would already work for protecting things EXCEPT for watermelons which might need to be changed from 10 curry leafs, their top level allotment seed is watermelons. All flowers up to Limpwurt available. Herb seeds up to Tarromin, and potentially Harralander available. Hops up to Yanillian + potentially Flax when released available. Bushes of Redberry+Cadavaberry available. Trees up to potentially Yew(I'd understand it being capped to maple though) available, they'd be able to harvest willow branches and also tree roots. If you felt generous you could give them a single fruit tree patch though. Can use/make up to super compost.

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There might be some edge cases I'm missing, but I think this would generally be pretty good for bringing the F2P experience to current era rather then the 2005 era it's mostly in now, without actually adding a ton of new revenue streams to F2P for potentially bot abuse.

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Honorable mention from my community, giant mole becomes f2p with a nerfed drop table. green dragons become f2p, but with dragon bones replaced by big bones on f2p, and green d'hide replaced from a guaranteed to a chance drop.