  We discuss the general framework for the construction of new models
containing a single, fermion number zero scalar leptoquark of mass $\simeq
200-220$ GeV which can both satisfy the D0/CDF search constraints as well as
low energy data, and can lead to both neutral and charged current-like final
states at HERA. The class of models of this kind necessarily contain new
vector-like fermions with masses at the TeV scale which mix with those of the
Standard Model after symmetry breaking. In this paper we classify all models of
this type and examine their phenomenological implications as well as their
potential embedding into SUSY and non-SUSY GUT scenarios. The general coupling
parameter space allowed by low energy as well as collider data for these models
is described and requires no fine-tuning of the parameters.
