Many, many things. Most notably:

* Support 10-bit addresses. At this moment, they are supported nowhere 
* Change the i2c modules to keep the namespace clean (EXPORT_SYMBOL does not
  work for us; many static declarations need to be added; better ask Simon
  Vogl first)
* Make it SMP-safe: there are no spinlocks yet. They are needed at many
  places, probably (everywhere where global vars are accessed).
* Extend sensors.c with a file /proc/sys/dev/sensors/chips, which lists
  all directories under /proc/sys/dev/sensors with their SYSCTL id
  (needed for good sysctl access).
* Write smbus_access_i2c in smbus.c
* Make lm78.c detect 'double hits', (same chip connected to both SMBus and
  ISA).
* Better lm78/lm75 detection; insmod-time paramters to set addresses 
  (difficult, because of all i2c busses!).
* Registrate which i2c addresses are used (a la ISA) for each adapter.
* Rename i2c-proc.c to i2c-user.c, and extend it with /dev entries;
  or, create a better i2c-dev.c, that understands SMBus commands.
* Write all other drivers
* How to make drivers detect thing *not*; for example, if a Winbond is 
  present, the LM75 driver should not try to access its simulated
  LM75 chips (in this case, simply disable them - but what if the LM75
  driver was loaded first?).
* Write a userland-library for SMBus/i2c access (through the /dev interface)
* Write a userland detection program for busses (clients are less important;
  but perhaps needed too).