I want to engineer a wide variety of plants, suited for various climates and microclimates, efficient at bacterially symbiotic internal production and toleration for concentrated alcohol - for energy. Maybe develop the GuinnessTree along the way for fun. The serious purpose, though, would be to make the energy equation such that the heat required for prodigious distillation of pure ethanol could be bootstrapped. Note: better keep a good firebreak around each so-altered forest, field or pond.
And for our next trick, electrically conductive plant structures, which of course with further elaboration on photosynthesis become palm trees, pond scum, and roof moss you can plug your portable nanoassembler into for power. Very handy indeed.
Now, having a friendly miniature bengal tiger in your living room, or a wooly mammoth on your farm, or becoming ourselves capable of running faster than a jaguar, or orders of magnitude more intelligent, might be all in good fun eventually- but why not be practical first before we play. Why not meet every energy and food need in 20 years or less. All this competition for resources, division of wealth, hunger, irritability, jealosy, etc. has been such a drag. Let's get cracking, shall we?