Given that Air France is apparently running into difficulties operating on routes to the continent from LCY on which Star and Oneworld already have a strong foothold, a high-yielding niche route certainly would be a good option if they cannot find suitable employment for an Avro RJ85 on those slots.
With a decent schedule form JER, a JER route would allow good - and most importantly - quick connections to places like AMS, RTM, GVA, PAR, SXB, EIN, NCE, LUX, DUB, EDI, MAN or DND on top of the local traffic.
If a Cityjet service would replace the existing daily VLM flight, we would be looking at only 50 additional seats a day. However, the big question really is how long 30 seater flights into LCY will last. Given the high fees, the economics must be quite terrible, and with a certain percentage of connceting traffic, yields will be even poorer unless AF is able to charge sky-high connecting fares from JER to the banking community. It will probably also depend on the fate of the existing LER-LHR link. IIRC from the Jersey-thread, there are rumours that it might face the axe.