  Extensice Air Shower (EAS) arrays are survey instruments able to monitor
continuously all the overhead sky. Their wide field of view (about 2 sr) is
ideal to complement directional detectors by performing unbiased sky surveys,
by monitoring variable or flaring sources, such as AGNs, and to discover
transients or explosive events (GRBs). With an energy threshold in the 100 GeV
range EAS arrays are transient factories. All EAS arrays presently in operation
or under installation are located in the Northern hemisphere. A new survey
instrument located in the Southern Hemisphere should be a high priority to
monitor the Inner Galaxy and the Galactic Center.
  STACEX is the proposal of a hybrid detector with ARGO-like RPCs coupled to
Water Cherenkov Detectors (WCDs) mainly to lower the energy threshold at 100
GeV level.
  In this contribution we introduce the possibility of improving the low energy
sensitivity of survey instruments by equipping RPCs, which were proved to be
optimal detectors at 100 GeV energies by the ARGO-YBJ Collaboration, with WCDs.
An EAS detector with high sensitivity between 100 GeV and 1 TeV would be a
valuable complementary transient detector in the CTA era.
