You have some great ideas with these Carthaginian and Roman sets, Rhalius, and I like Tim's variations on them too. Although the Roman theme is now gone, a few stand alone regular releases and Add Ons could be offered, just as Playmobil has done with their Western theme.
I really think the armored Carthaginian elephant would be a big hit with children and adults alike. Place a mahout, an archer, and Hannibal on it and the set would be a winner. A handful of diverse Carthaginian warriors with a small section of wall (with larger sections available as Add Ons, as Tim suggested) would be nice too and I think would sell well. The docks and merchant ships might be less successful and less likely to be produced, but it's a great thought.
When the Roman theme was introduced I was overjoyed and I really hoped Playmobil would expand upon their initial offerings somewhat with a villa and a watchtower, perhaps as Add Ons, and possibly a forum, complete with a senate building on one side, a bath on another side, market stalls on another side, and a speaker's podium in the center, as a large regular set. At the very least, I would have liked to have seen some additional klickies.
In addition to the above, my ideas for the second generation Roman theme were/are the following:
-green trimmed Roman ship
-green quadriga with a green trimmed officer (maybe red and white Add On quadrigas so we could have a proper four-color chariot race)
-green auxiliary soldiers with a new oval shield printing and a cornicen
-black Praetorian guards, perhaps with gold armor, featuring a new scutum shield printing and an aquila, along with an officer
These sets could still be offered today as Add Ons.
I have other ideas for other ancient themes, but that's for another post.