Green chemical modification of cellulose presents a unique chemical challenge, especially from the vantage point of sustainable development that is favored by the use of wood fibers, heterogeneous conditions, and reactants and solvents of biobased relevance. However, heterogeneous conditions imply that cellulose is a supramolecular assembly whose composition and build-up depend on the initial source and pretreatments. Also, understanding reaction outcomes is accompanied by inherently challenging characterization. The key question is how we should design our reaction systems to achieve customizable and green functionalization of cellulose under heterogeneous conditions. To explore this, we selected never-dried high-content cellulose fibers (>96 % cellulose) as the substrate for the modification with three relevant biobased reactants (succinic, maleic, and crotonic anhydride), with BBIL-AcO as a biobased reactivity promoter. The reactions were performed under either high fiber swelling (basic) or low fiber swelling (acidic) heterogeneous conditions, and the outcome was analyzed in detail. The results unravel clear design strategies for controlling the reaction outcome during the green heterogeneous functionalization of cellulose and present clear synthetic strategies for using cellulose as the key substrate in the next generation of fully biobased and green materials.