Triaxiansilk.com

The would-be Season 5
Possible design of a 22nd century Kzinti starship
The Enterprise was due to visit Stratos, showing the split-up of its society as it is depicted in TOS: "The Cloud Minders"
Revisiting the mirror universe and possibly featuring Hoshi Sato being empress of the Terran Empire was discussed for Season 5.A fifth season of Star Trek: Enterprise was never produced, since the show was officially cancelled on 2 February 2005. The producers, however, had already devised numerous plans for future seasons, which could have started airing by September 2005. Most information is based on comments by producer Manny Coto.

A Kzinti episode had been suggested as a prequel to TAS: "The Slaver Weapon", which progressed as far as a "rough rendering" of a Kzinti starship, commissioned by writer Jimmy Diggs. The story was titled "Kilkenny Cats." [1]
Manny Coto has stated that, had the series been given a fifth season, the recurring character of Shran may have joined the crew of Enterprise, as an "auxiliary or an advisor" [2] [3]
At the 2009 VegasCon, Coto suggested that two big themes of the season would have been to show "origins of the Federation" and "whispers of the Romulan war". Consequently, the Romulans would be the major villains of the season, although other species may have appeared in the mini-arcs. Brannon Braga noted that he and Rick Berman had considered making "Future Guy" a Romulan. [4]
Plans existed for an episode showing the construction of the first starbase, most likely in the Berengaria system. First hints to that episode were already given in "Bound". [5]
Enterprise was due to revisit (actually previsit) the cloud city Stratos on Ardana showing the formation of the two castes seen in TOS: "The Cloud Minders". [6]
Furthermore, revisiting the mirror universe, which had already been shown in "In a Mirror, Darkly", and possibly featuring Hoshi Sato being empress of the Terran Empire had also been discussed. At the 2009 VegasCon, Coto revealed that one idea was to spread four or five episodes through the season, as a kind of "mini-series inside a series". He stated it was his "big regret" at not getting a chance to do so. [7]
Judith and Garfield Reeves-Stevens pitched a story with Alice Krige as a Starfleet medical technician who makes contact with the Borg from Season 2 ("Regeneration") and becomes the Borg Queen. [8]
Writer/producer Mike Sussman hoped to have T'Pol finally meet her father, and reveal to the audience that he was in fact a Romulan agent who had posed as a Vulcan officer prior to faking his own death. The suggestion that T'Pol was half-Romulan would have shed light on her affinity for Humans as well as her interest in experimenting with emotions. (Information provided by Mike Sussman)
In a 2009 interview, Welsh writer/producer Russell T Davies, showrunner of the British series Doctor Who, said that he had considered doing a crossover between Doctor Who and Star Trek, but the latter was canceled before the idea could be pursued.[9]
A design for a 5th Season refit of the Enterprise is planned for inclusion in the 2011 Star Trek: Ships of the Line Calendar.

Zane Gray wrote:Original NX-01 designer Doug Drexler reveals that if Season 5 had gone forward, we'd have seen a refit/upgrade of the ship. It was already being planned... and now Drexler has finally completed work on it for the forthcoming Ships of the Line calander. Best of all, he's just revealed the plans for the new design on his blog. Think of the story possibilities...

http://drexfiles.wordpress.com/2010/04/25/nx-01-refit-diagram/