I have in my series a group called "Chimeras". They were once normal humans but were exposed to an artificial gene-editing retrovirus known as the "chimera virus".
The chimera virus is a synthetic virus that lacks genetic information of its own and serves as a vessel for introducing foreign genes to the cells of a host. Every batch/strain starts as a blank slate, but copies the first genome they're exposed to for spreading to all future hosts. In a controlled environment, the DNA fed to them is carefully selected for desired purposes. Should they get into an organism without prior treatment, or if DNA is added to them irresponsibly, they end up carrying a sizable portion of genetic information to spread to all future hosts.
Any strain carrying foreign DNA works its way through the host, spreading across the majority of their cells and stimulating growth. Physical changes start slow and minor at first, but given enough time and nutrients, their body becomes unrecognizable and inhuman.
In trying to keep it somewhat grounded, there are some specifics on how the chimera virus operates.
It's not a fast process: A complete mutation into a chimera takes a lot of time and food for their body to grow and build any new structures/components. Their appetite massively ramps up for a while to gain the necessary material. It's not an experience most would willingly undergo, as the process is very painful. Think less Ninja Turtles, more "The Fly" in terms of transformations.
It's not "Lego-genetics": There are no mix-and-match parts here. Someone who received lobster DNA or something isn't just gonna grow a claw, the rest of their body is going to slowly and painfully warp too. No major change can occur in isolation, it always results in the rest of them changing to some degree as well. Having the chimera virus carry majorly different DNA strands at once (like say, a crab and a gorilla) will always result in the host dying. The species are simply too different, and the incompatible simultaneous changes to their body would not be good for things like.... living.
I know this is likely beyond modern (and probably all future) gene-editing technology, but does it at least make some degree of sense?