Manny Coto, Mike Sussman, and JGRS have provided a lot of ideas for what may have come in season 5 over the years, some of those idea I thought interesting, others I was kind of iffy about (though, to be fair, I was also dubious about the idea when I heard they were making yet another mirror universe episode).
http://en.memory-alpha.org/wiki/Undeveloped_Star_Trek:_Enterprise_episodes
-The plan to have Shran join the crew (and presumably making Jeffrey Combs a series regular) is well known. I don't think anyone would've had a problem with that (except maybe Anthony Montgomery and Linda Park playing second fiddle to YET ANOTHER actor
). Seeing Shran adjust to life wth humans and learning about their culture (i.e. going to movie night) sounds fun. I don't think you have him calling Archer "pink-skin" anymore (which frankly probably woud've been for the best given the somewhat racist implications).
-I don't think the idea of visiting Stratos excited me as much as it did Manny Coto, but I'm sure they could've pulled it off.
-Probably the only idea I really didn't care for was JGRS' idea of having a Starfleet med tech (played by Alice Krige) who turns into the Borg Queen. It's too Earth-centric and makes little sense, since the Borg are on the other side of the galaxy at this point.
-I heard about the idea of having them encounter Flint and immediately thought Tim Daly would've been the perfect choice to take over his father's role (assuming they could afford him). I think you'd have to address the implausibility of a near-immortal human. Maybe he wasn't totally human, or was enhanced by some advanced alien race, for some reason.
-I admit to having some reservations about a Kzinti story, largely because I grew up considering the Animated Series non-canon. I admit to being skeptical that they could depict them convincingly anyway in prosthetics, let alone CG (furry CG characters are notoriously difficult to render).
-As much as I liked "In a Mirror, Darkly", I'm not sure about revisiting the mirror universe. I think DS9 ended up going to that well a few too many times and wouldn't have wanted "Enterprise" to repeat that mistake.
http://en.memory-alpha.org/wiki/Undeveloped_Star_Trek:_Enterprise_episodes
-The plan to have Shran join the crew (and presumably making Jeffrey Combs a series regular) is well known. I don't think anyone would've had a problem with that (except maybe Anthony Montgomery and Linda Park playing second fiddle to YET ANOTHER actor

-I don't think the idea of visiting Stratos excited me as much as it did Manny Coto, but I'm sure they could've pulled it off.
-Probably the only idea I really didn't care for was JGRS' idea of having a Starfleet med tech (played by Alice Krige) who turns into the Borg Queen. It's too Earth-centric and makes little sense, since the Borg are on the other side of the galaxy at this point.
-I heard about the idea of having them encounter Flint and immediately thought Tim Daly would've been the perfect choice to take over his father's role (assuming they could afford him). I think you'd have to address the implausibility of a near-immortal human. Maybe he wasn't totally human, or was enhanced by some advanced alien race, for some reason.
-I admit to having some reservations about a Kzinti story, largely because I grew up considering the Animated Series non-canon. I admit to being skeptical that they could depict them convincingly anyway in prosthetics, let alone CG (furry CG characters are notoriously difficult to render).
-As much as I liked "In a Mirror, Darkly", I'm not sure about revisiting the mirror universe. I think DS9 ended up going to that well a few too many times and wouldn't have wanted "Enterprise" to repeat that mistake.