Well a site which holds planning permission for a 60 storey, 188m tall tower has literally only just been put on the market. This project got to the stage of rebar protruding out of the ground, ready to rise before the recession hit, haulting the project indefinetly. With the location of the site being at the foot of Manchesters largest railway station, which is set to expand massively when the new HS2 lines get built, this site has massive potential to be THE gateway building of Manchester. Think of it as The Shard to London Bridge Station.
I presume one of the three developer giants in Manchester will snap up the site, either Renakar, Allied London or Bruntwood. If Renaker, possibly expect a larger tower than what the site currently holds planning for 
I will make a thread for this site shortly. 
There are also rumours of a 50-60 storey tower to the north of the city, which will add a peak to the forming Greengate / Northern Quarter cluster, the current tallest in the area, which is in the piling stages, is the 130m tall Exchange Court Tower; as seen in the thread linked below.
I have a feeling that the rumoured tower will be on a derelict site by the river, although we will have to wait and see.