i would expect raqibul and shakib to emerge with the best test records among the present lot. which means a solid but unexceptional average over 35.
mashrafee will be our first bowler to cross the threshold of 200 test wickets. i hope he finishes with a haul of over 250 wickets with an average under 35.
as for ashraful, making any predictions about his performance for the next game is fraught with danger, let along assessing his career average. if he does play for 10 more years, ash should end up with more than 120 tests. if he can finally find some consistency in his batting he can still end up with a 35ish test average.