I expect this suggestion will be met with howls of derision, but I honestly think it would be healthier for the game if it were implemented.
I think that the option to Tenaril should be limited to the first city founded on any account. I think that the option to Tenaril ANY city should be removed after the Exodus spell has been used for any city on the account.
"Terraforming" with Exodus followed by Tenaril is a popular option, but personally I think it fails the "fun" test. Yes, it can make a "good" square out of one that previously wasn't so good, but I think the pressure especially on new players to do this process early in their game careers results in people getting bored trying to build up level 20 warehouses and increases attrition.
Yes, this is a self-imposed misery, and rational people could choose to Tenaril in the usual way, but I think new players are often led by extremely well-intentioned established players to feel that Exo-Tenaril is the best option in the long term.
The ability for established players to create alts that then terraform a city that can be captured also eliminates some of the trade-offs that would otherwise occur in some regions (such as arctic and desert). I think this makes the game less interesting and results in a lack of creativity and diversity in game approaches; instead terraforming has become a one-size-fits-all answer.
My alliance makes extensive use of terraforming, and I would strongly consider using it myself. I don't consider it somehow "wrong." However, I do think it has negative effects on new players and existing players and that the game could be far more interesting without this option.
My general hope for Illy is that it should be a game that encourages many ways to approach problems, many paths to success, and also that the best solutions should be those that require some level of thoughtfulness. Terraforming with Exo-Tenaril fails this: It's a one-size-fits-all solution that anyone can do if they are willing to click enough. If Illyriad is a sandbox, then Exo-Tenaril is like a mold that creates a perfect -- and exactly the same -- copy of a castle upon being filled with enough sand and then upended. This may create an attractive scene but not a particularly interesting one.