It's possible, it would be interesting to see what uses people could find for it in a classic Amiga.
Audio applications have already been suggested, XENA could give us a 16/24 bit sound card, speech recognition perhaps, can XENA implement an Ethernet port?, how about an SD card interface, USB port, it could drive a pair of ultrasonic transducers to give us similar functionality to that MS device whose name escapes me at the moment. EDIT: Kinnect.
XENA could give us a multifunction card that could evolve as people developed the software to implement new interfaces and ideas.
Software support would evolve faster if more people were trying to use XENA.
But XENA is not that. It is just renamed XMOS chip. XMOS chip itself is capable to do many tasks and projects you suggested are more or less possible. But it would be just piece of electronics.
Instead of XMOS chip (XENA) also FPGA could be used or off the shelf network interface chips. They are relatively cheap, was it XMOS, FPGA or some other chip. But designing Amiga expansion card with any chip is expensive. You need good knowledge of Amiga hardware (which kind of knowledge is fading) and you need someone to write drivers so your favorite TCP/IP stack can use your new ethernet card.
It is also reason why there are no Xena applications for X1000. It is easier to buy supported sound card from ebay than design one yourself from the scratch including drivers and electronics work. It could be fun but those who find it fun are rare.