This review analyzes the effect of various natural fillers on the mechanical properties and process stability of polylactic acid based composites produced using Fused Filament Fabrication (FFF) 3D printing technology. The study synthesizes recent findings involving a wide range of bio-fillers, including hemp hurd, date palm fibers, spent coffee grounds, lyocell, and lignin. Key research highlights include record-high tensile strengths for 20 wt% hemp hurd composites (70 MPa) and a significant doubling of stiffness (7.1 GPa Young’s modulus) at 40 wt% loadings. Furthermore, small additions of hemp (2.5 wt%) and date palm fibers (5 wt%) were found to increase tensile strength by 29.6% and 6.53%, respectively, while simultaneously offering weight reduction. The review identifies critical 3D printing challenges, such as moisture-induced porosity and nozzle clogging, and proposes a 'printability window' recommending particle sizes between one-fifth and one-half of the nozzle diameter. Chemical modifications, specifically lignin esterification and the use of coupling agents, are highlighted as essential pathways to improve interfacial adhesion and overcome the inherent brittleness of PLA. Finally, this work formulates general design guidelines for engineers to optimize filler loading and printing parameters, supporting the integration of agricultural waste into the sustainable circular bioeconomy.