757 is slightly limited in long range. 767, if you can afford them, are rather nice, and the -400 version kicks ass in terms of flight cost per seat(but you'd better fill them). A321 is not far in terms of capacity, but costs you one arm.
If you need less range and capacity, the 737-900, at the other side of the 757 spectrum, is a sound choice, though prohibitive. Or even smaller, the MD90-55, which is not THAT costly. Though check if it fits your strategy.
TU204 is no more manufactured, I fear. It's the nearest equivalent.
And there is no new plane covering the niche to arrive. The C919(156 seats) is probably too small for your needs, and will arrive in more than 10 years. So sticking to 757 is maybe not a bad choice.