That question can be seen equivalent to asking "How many computers will we need in the future?" in the 1950s. Could be 5 could be the current 5 billion or how many we have to day. Maybe someone more involved can say more about it. In theory possebilities are enourmous like currently healing HIV, personal cancer therapeutics, creating of custom crops (that currently do not necessarily count as GMO, mind you), enhanced model organisms. More down the road are deextinction of species, extinction of invasive species (rodents, zika, mosquitos, tilapia, humans ;). Targeted editing of embryos with herited disease. It has been said that if only 10% of what has been proposed will be feasible, we have a huge game changer.

Whether this is playing god I do not dare to say (Last member of my family was babtised 80 years ago). When people invented the airplane people said that the sky is god place and they should not go there. Craig Venter was dubbed God 2.0 many years ago. Still, its just another new technology. In the 1940/50s antibiotics were a huge improvement, today its just what we are used to. I think I will get along with it, what you think depends on you.

An evil person could do many things, like target ethnic groups based on their genetic haplogroup with an infection similar to the one currently being tested to eliminate mosquitoes, making all of their ancestors infertile in a few generations without their knowledge. In the future we could see something similar to the arms race, a sort of cyborg race with governments rushing to come up with way to make cryptographic DNA or mind transfers to protect their citizens from genetic terrorists.