  The High Altitude Water Cherenkov (HAWC) high-energy gamma-ray observatory
has recently been completed on the slopes of the Sierra Negra volcano in
central Mexico. HAWC consists of 300 Water Cherenkov Detectors, each containing
180 m$^3$ of ultra-purified water, that cover a total surface area of 20,000
m$^2$. It detects and reconstructs cosmic- and gamma-ray showers in the energy
range of 100 GeV to 100 TeV. The HAWC trigger for the highest energy gammas
reaches an effective area of 10$^5$ m$^2$ but many of them are poorly
reconstructed because the shower core falls outside the array. An upgrade that
increases the present fraction of well reconstructed showers above 10 TeV by a
factor of 3-4 can be done with a sparse outrigger array of small water
Cherenkov detectors that pinpoint the core position and by that improve the
angular resolution of the reconstructed showers. Such an outrigger array would
be of the order of 200 small water Cherenkov detectors of 2.5 m$^3$ placed over
an area four times larger than HAWC. Detailed simulations are being performed
to optimize the layout.
