As nice as more crewmates might be, there are 22 already (24 if you count Varmint and Siegwulfe, but they're more pets) Among them there are 5 who are modified humans and 3 who are androids and 1 is an AI-GreyGoo. The rest are aliens though, there is a Dzaan, an Ausar, a Suula, a Galotian, a Milodan, 2 Kaithrit, a Kui-tan, a Gabilani, a Half Ausar Dog-morph, the Pexiga, a Raskvel, and a Sparadat. More species in the crew members than any Mass Effect game.
I have trouble keeping track of them all as it is, I'd rather they focus on more development for those already on board than add more.