Why specifically Sembawang and Queenstown? Between this two
rather remote towns, MRT would be way faster. Plus, there are other
links to other towns from both Sembawang & Queenstown (e.g.
Choa Chu Kang) that could have been improved.
Nonetheless:
Actually, Sembawang is only one of the towns where the popular
routes to Buona Vista / Queenstown area originate
from. 
The other towns are Tampines, Sengkang, Yishun, Jurong East,
Bedok and Woodlands. 
However, it is only Yishun, Jurong East, Bedok and Woodlands
that have direct bus connection to Buona Vista /
Queenstown. 
Sembawang, Tampines and Sengkang do not have direct bus
connection to Buona Vista / Queenstown. 
Somehow, Buona Vista / Queenstown is a popular destination in
the morning. It is the second most popular, after the city
area. 
After Buona Vista / Queenstown, the other
popular destinations are the Eastern-region industrial areas,
AngMoKio-ToaPayoh area, Jurong East (International Business Park)
and the Nanyang Technological University . All these can be found at
www.beeline.sg. 
It may be better to suggest bus routes to cater the travel
routes which have the high demands.
Whereas for the route suggestion of yours, the connection is
somewhat there, but I feel that it could be faster if we skip Bukit
Panjang. It would
be better if the route plies North Buona Vista Road along the
circle line, because that's where the popular route from Sembawang
to Queenstown leads to. If possible, avoid Ayer Rajah Bus Park
because I do not think that place is for bus services to
terminate. Kent Ridge Bus Terminal may be a better
option. Still, the route is quite long, at
least around 25.5km. 
but at least it's something. currently, I think the MRT and the existing
bus services are good enough for travel between Sembawang and the
areas along North Buona Vista Road. if there's ever a need for additional
capacity, we could use this suggestion.