Coincidentally, Sengkang and Taman Jurong currently do not have
express bus to the city.
Maybe they feel that combining two express routes into one is
more economical, that's why they come up with 500.
("I have Sengkang express. I have Taman Jurong express. Uh!
500." 
)
Hopefully, this 500 can also reduce travel time between Jurong
West and Sengkang.
Currently, it takes around 1.5 hrs to travel between JW and SK.
If 500 can run from JW to SK under 1 hr, that would be great. In
fact, I think 30 mins is possible during non-peak hrs...
But travel demand between Boon Lay/Jurong West and Sengkang is
limited, unless you're talking about NTU. If these rumours are
true, I think svc 500 would be mainly for JW - city and Sengkang -
city, rather than JW - Sengkang. If skipping the city area, the
most likely route may be Sengkang - Bt Panjang/CCK - Boon Lay.
Let's just wait and see...
Other than new svcs for Sengkang, we should see new svcs in
Tampines (from the bus int extension, possibly to St 86, Tampines
Concourse, Tampines North Ind Est and/or St 92 Ind Est), and
new/amended svcs in Tuas when TWE MRT opens (route amendments to
improve connections to the new MRT stns, maybe a new svc for Tuas
South).