I remember when in Fall from Heaven cities got able to give units build there additionnal promotions based on special buildings or ressources available, like the "<material> weapons" promotion.
The problem with those promotions was that they easily made existing units sort of obsolete once the new promotion became available, forcing the player to build his army again if he wanted to get the new promotion.
This was solved in FfH by having any unit moving into a city automatically get any of the available upgrades/promotions that this city would give to newly built units there.
I have been hoping for something like that to be implemented in Elemental for a long time, and with Derek managing the game now, I was thinking that it would be fast to come, but unless I missed it in a patch this is stilll not the case, so please Derek, could we get the same in Legendary Heroes ?
If a free upgrade just by moving on the city is too much, needing to take city time to upgrade the units, or needing to pay for the upgrade (an option that could get available when stationned in the city of course) could be alternate solution.
Thanks for reading my post.
Edit: also, as suggested by thadianaphena, this would also prevent some buildings options in Conclaves and Town that boost units created in the city from behing bad choices compared to others, are currently if you want such bonus you will certainly build your units in a fortress instead.
With a "train in city" upgrade option like described in my second post, such buildings could be used for existing units in addition to the fortress bonuses, making them a viable choice for a player wanting a more elite military.