longterm i would expect to see A320 family or EMB195's replace the 734's at LGW so that there is a common fleet with either LHR or Cityflyer, after all Cityflyer have have now got most of them so should be in a fairly good position to evaluate the EMB's.
I think the replacement will be Airbus. As Airbus crew are dual based at the moment (LHR/LGW). LCY/JFK airbus crew come from LGW.
I think the 734's will be replaced by the A319. BA may also add a small fleet of A318's for routes like LUX, MAN, GLA and EDI.