Crazy (likely bad idea) for Fantasy Role playing Fun. Pick a fantasy RPG and replace major rules components from different games and mash them together to make a fantasy campaign.
Let's take original D&D
Use the ability scores and generation method from The Fantasy Trip
Use Runequest battlemagic spells for PC-magic.
Use 7th Edition Tunnels and Trolls weapon statistics.
D20 D&D saving throws.
We'd have a little tweaking to do on our hands.
Those ability scores, easy to keep track of, PCs aren't going to have many modifiers. Maybe lump together a couple of the D&D scores functionality.
Runequest battle magic has lot's of spells but needs a POW stat/magic points. Do we make a new stat or use one of the three main characteristics? Let's use strength. Maybe spell casting classes get some bonus magic points equal to their level. No one has a big flash-bang combat spell. I'd go with STR as POW and bonus Magic Points equal to level for spell casting classes (STR + level, not each level)
Tunnels and Trolls weapons do a lot more damage then D&D weapons. Lot's of fights are going to be quick. Boost PC-type HP the same amount as their STR score, they are still goign to be soft and squishy just not as badly so without the boost. T&T weapons have ability score requirements. Don't meet one stat then -2 to hit and 1/2 damage with that weapon, don't have either you can't use the weapon. Except for weapon stats we are still using D&D combat rules.
Let's Throw in a parry rule, which the weapons of T&T sort of go along with. A parrier gets to subtract their damage roll for the foes damage roll, parrying gives up a chance to make an attack in a round.
D20 style saving throws aren't hard to lift and adapt. We've got classes and level already, just remap each save to one of the three ability scores.
There you go, an "original" set of rules for a fantasy rpg campaign.