I once wanted to run two RPG system trees, one hard and realistic, the other pulpy and light.
The two system families were;
Savage Worlds for light weight and pulpy.
The d100 family of games, including the Call of Cthulhu, Runequest, Openquest and D100 lite branches.
The two systems seemed to mirror each other for available resources in the 5-6e CoC and SW Deluxe editions. Want Rome, Weird War 2, Cthulhu horror, Western, even Sci-Fi and Supers? They both provide.
The idea was to fit the mood, but I usually shied away from SW in favour of one or more d100 versions for each theme. The assumed player friendliness of SW was actually harder to teach and play than a straight firward D-percentage style game.
The reality is, there is more genuine player fear in a simple game of Vikings of Legend than a horror themed SW game. Real fear comes from an awareness of random and uncontrolled peril, not just thematic art. In VoL or WHFrpg your character can loose an arm just like that. SW, a lot like many d20 games seems to lose that feel in the abstract and soft mechanics.
The prime example was Achting Cthulhu. This one came with both (and optionally other) systems as the mechanics for their adventures, but without fail, I found SW a thin game in this space, even less meaty than the board game. CoC was the real thing, scarier, more dangerous and more visceral.
There were some games however that I felt could or should have jumped the divide.
Luther Arkwright, a Mythras based game I feel would have been a better fit for SW, being a comic book based, pulpy, multi dimensional game. SW would have felt like a good fit, been more versatile and pick-up flexible and many of the SW world books could have been folded in seamlessly.
On the other hand, Solomon Kane, a dark, perilous, horror themed game written for SW, is a perfect fit for any number of d100 systems, CoC in particular. The Cakebread and Walton Clockwork and Chivalry/Cthulhu games maybe even better. Even the Warhammer or Zweihander games with little modification. In SW, it just lacks the base mechanics, the darkness and mechanical hardness to feel right.
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One of the reasons I think Savage Pathfinder is such a good fit is the massive amount of content a d20 game requires to give you the illusion of freedom. This huge array of blinkered choice is a burden forced on it by its very restrictive nature. Rail-roading players into strict class envelopes always leads to envelope pushing exceptions, but the envelope is still sealed and it needs to be for the paradigm to work.
Savage Worlds offers vastly fewer core options, but has effectively no limits within those. Classes are a choice, not just which, but whether to or not. Spells, cut down from over 600 to less than 50 have more options built in, so they feel more like controlling chaos than strict recipes and are infinitely easier to implement (and have no fixed slots!). Feats rather than add-on exceptions and special actions are character defining edges and hindrances.
I never thought I would say this, but I think SW-PF is even better at not being a traditional d20 game than 13th Age.
It manages to streamline the core system, adds in modern RPG tropes like 13th Age’s “one unique thing” through more defined edges and hindrances and even adds elements d20 games cannot.
The key to me is flexibility. Pathfinder-SW has more with less overhead.