*Even more distant African chuckles*
Sorry bros, lol...
I always feel a bit bad about posting architecture/building-set suggestions... Of course I never expect you to do all the work, but I'm aware that in reality, if the idea is liked/accepted (by you ), like 90 to 100% of the work would fall on you...
So I'd like to emphasize that I'm just someone who likes to brainstorm and fantasize about the perfect RTS. So I just put my ideas and opinions out there... It's up to you and the greater community to see if it's actually feasible/desirable.
I'd also like to emphasize that the "only" structures that would need to "evolve" from phase to phase are houses and the CC (other structures are already tied to later phases anyway). It would be more than enough to generate that feeling of evolving from village to city. The subconscious feeling would a greater sense of emotional attachment to your settlements (because you saw them grow/evolve/mature).
As Lion said, some if it can be done like:
Although the standard Carthaginian houses evolving in to those pretty apartments is ideal in my opinion.
Another thing is that, if it's not tied to individually upgrading CC's, it would have awkward effects... Like, if you reach city-phase, and all CC's upgrade simultaneously, you could end up with a highly ornate CC, and a pretty city apartment next to some fields in the middle of nowhere, which would also break immersion...
I think individually upgrading CC's, and visual differentiation between phases go hand in hand, and would make 0AD even more awesome...