You can find the long term development plan at this link
Perhaps the most relevant section for this discussion is on pg 37:
A number of existing rides would require replacement as they reach the end of
their planned lives. This is likely to include Ripsaw, the Flume and Blade. In
addition two large attractions/rides are likely to be developed in 2016 and
2019 (one of which may include a ride across the valley between UG Land to
the Forbidden Valley) resulting in approximately six new or replacement medium
to large attractions.
It's interesting that there are a few rides mentioned here which give the potential for a number of new attractions by the end of this decade. What is also intriguing is the fact that they only call them "medium to large attractions" therefore not specifying coasters or flats, or what target audience these attractions would be aimed at.
I would hope that we will see some new flats, as I think the park needs it to keep the balance of attractions in the park. What these will be however is extremely difficult to guess as a lot will depend on what the marketing department determines will attract visitors to the park.