Monarch's demise has taken 35 aircraft out of the mix. That includes 4+ at LTN and 9+ (?) at LGW. That's a fair number to replace, and LTN/LGW will be viewed more favourably by most airlines than SEN.
We can expect some replacement of MON capacity from EZY, TUI & TCX at both airports, and perhaps BA at LGW and Blue Air at LTN. FR may be in a position to introduce new capacity by next summer, but are likely to favour STN.
Under these circumstances, I can't see SEN attracting any significant new operator in 2018.