Besides simply extending the roof trusses, you could
1) Reinsulate the 3.5 inches between the roof trusses with something of higher performance, like spray in polyurethane. That is about twice the insulation value of fiberglass and contributes substantially to roof/joist solidity.
2) If twice the insulation is not enough improvement, instead of extending the trusses, add a layer of 2x4s (or 2x6s, etc.) so that the result with the roof trusses are a cross hatch pattern. Lumber is not an excellent insulator (it ranges anywhere from 35% of fiberglass to 90%), so minimizing the amount of lumber conduction from the roof to the ceiling is a worthy goal. I am unsure of how to tie those together, but maybe something like this (bottom second from right) would work. This approach may require consulting with a structural engineer to make sure the roof is strong enough.
3) Insulate the new layer.