I don't think the OP is saying to screw over new players. I think she's saying if the DF decides to assign her to a party with a level 17 player, or decides to assign such a player to her incomplete pre-made party, then she doesn't mind if it ends up picking Sastasha (For example). However if all the players end up being over level 40 (for example), then it should be a random 35+ dungeon.

Seems fair enough. I don't know how DF's algorithm works for deciding who's in what party so this might be quite tricky (but as with anything, not impossible, just means if it's tricky it won't be implemented for a long time).