TCV experiments and novel theoretical studies have addressed key questions regarding the operational space of negative triangularity (NT) plasmas in view of an L-mode NT reactor, in terms of stability, performance properties and core-edge integration. We show that reducing the top triangularity (δtop) to more negative values induces an H-L back transition, confirming the direct dependence between H-mode existence and access to 2nd ballooning stability region. We also show that the X-point δ can prohibit H-mode access if sufficiently negative. Using these conditions to stay in L-mode, we show the sustainment of a stationary high βN NT plasma, with H98y2 > 1, using real-time β control with NBI as an actuator. This plasma has Ti > Te over the whole profile. Gyrokinetic simulations show that the benefit of NT might be lost for aspect ratio <2.5, in the TEM-dominated microturbulence regime and that the improved confinement is primarily due to the non X-point triangularity (top δ in single-null down divertor, SND). Global gyrokinetic studies also show that this improvement does not change with machine size (independent of ρ∗). These studies as well as other results presented at this conference allow us to propose in the conclusion a procedure for predicting and optimizing NT reactor plasmas which can be tested in present NT experiments.