__________, the situation has completely transformed over the past few years.

'Predominantly' means mainly or for the most part, and serves as a discourse marker emphasising that the transformation has been substantial and dominant. It fits well with 'completely transformed' to reinforce the extent of change. 'Somewhat' would contradict 'completely', 'Believably' is awkward, and 'Thus' indicates consequence — none fit as well as 'Predominantly' to qualify the transformation.