Great LAMPOIL OP! Well essential news for the OP is that DTG are not doing any more steam locos for TSW nor so far as we know are any third parties. And as regards the specific suggestions, an A4 LNER Pacific would just add another large passenger loco when what's needed is something mixed traffic. And a UP Big Boy completely out of the question when the limited steam physics which DTG are not going to improve, can't even do a decent job with relatively simple UK locos.
I see this is OPS first post. The first couple of posts someone makes have to be verified by a moderator before they are posted. Really makes you think why a moderator allows spectacularly undetailed suggestions like this...