The only thing I have personal experience for something like this is the old D20 Modern, which is based on the same general ruleset as D&D 3.5 and PF1e with a bunch of little tweaks and different base classes and such that are based on a focus on skills and traits associated with a particular ability score. With a goal to being more flexible it’s designed to be adaptable and generally expected for characters to do at least some multiclassing both with multiple base classes then to prestige classes that focus on things like Soldier, Infiltrator, Celebrity, etc. There’s gear and modern equipment with rules for stuff like car chases and gunfights. It also has a full sourcebook dedicated to a campaign setting called Urban Arcana with rules for adding magic and other fantasy elements. Spellcasters generally feel a little bit weak because all classes only go up to ten levels and spells to 5th (with all casting coming from prestige classes so you need a few other levels first), with the general structure to go up to level twenty or higher by just adding more classes to your build after maxing one out. A lot of the spells are more useful for things other than direct combat and there are rules for doing things like planting an attack spell into an email or such. More powerful magical effects are achieved via rituals, which often involve occultish stuff like getting a bunch of people to chant around a big arcane sigil on the ground to add more power to a lead caster and can do some pretty crazy stuff if you get enough people with high enough bonuses to the appropriate skill (mystic lore or something, been a long time since I’ve actually played it).