I was thinking of a couple of unit upgrade paths that we should include in the next revision.
First, I think
Modern Infantry should be allowed to upgrade to
Special Infantry as well as
Mechanized Infantry. The current main city defense line goes Warrior-Slinger-Archer-Longbowman-Arquebusier-Musketman-Rifleman-Infantry-Modern Infantry and then breaks the pattern with Mechanized Infantry, because as a Tracked unit, Mech Inf cannot gain City Garrison promotions. Also, I would probably never use
Walker Mech as a main city garrison unit because of its +2
cost per turn. Special Infantry is just as effective without the extra gold per turn. I do use Walker Mechs as offensive units, as I generally have some units that get upgraded all the way from Swordsmen. Those units get disbanded once the last AI is eliminated, so it's not that bad. (I generally prefer a mostly infantry army as my stacks need to stay together to protect the siege units.)
Second, none of the Doomsday units upgrade at all, so that leaves a lot of old units on the build lists. I think
A-Bomb should upgrade to
Tactical Nuke or
MRBM (MRBM offers pure upgrades over the A-Bomb) and
ICBM should upgrade to
Hydrogen Bomb (both are unlimited-range nukes, but Hydrogen Bomb has nuke radius 2 over ICBM's 1).
On a related note, is it necessary for the A-Bomb to be a
Bomber unit? All the rest of the nuclear units are
Doomsday. The Bomber classification seems to only give it access to Air promotions (which are not all that useful as the unit is consumed on use) and allow it to conduct Air Strike and Recon missions - but with only 1 Strength, an Air Strike won't do much and I doubt anyone would want to use an A-Bomb for Recon.