The most important parameter on the mechanical properties of the membranes is the Polymer Concentration. But, enhancing the polymer concentration decreases the flux (LMH). So, it is a trade-off between mechanical properties and flux.
Generally, open-structure membranes which have big macrovoids or finger-like pores are more fragile than tight-structure membranes. Sometimes, the presence of plasticizers (additives) can improve the tensile strength of the membranes but the most of plasticizers cause macrovoids.
Therefore, it seems you should find the additives which make denser structure or sponge-like structure such as TiO2 nanoparticle. And maybe you should search about the small-molecule additives. Another important parameter is the additive concentration that affect on the final structure.
Maybe there is not a novel additive but for novelty, you can use two kinds of additives at the same time.