Agree with Mako, I think a family-thrill coaster is the way to go. If and when the Buffalo leaves I'd look to replace that solely with a family coaster. A gap between Shockwave and Accelerator would be ideal.
To complete the coaster line up I'd also add something similar to Legoland's Duplo Coaster to Thomas Land, a first coaster for toddlers, a Baby Thomas coaster or something like that.
That would leave a decent line up;
*Baby Thomas Coaster
Troublesome Trucks
Buffalo
Accelerator
*New Family-Thrill
Shockwave