Lessons order: Perfect/Preterite
Based on my own impression, I would propose to slightly rework introductory lessons "Perfect 1" and "Preterite".
Why? It is said that in colloquial German, the perfect is the one commonly used, while the preterite is more used in official language. At the same time, some verbs -- such as to be, to have, modal verbs -- are always used in preterite. But the Preterite gets introduced shortly (6 lessons) after Perfect in the tree, before one gets accustomed to the usage of Perfect. As a result, personally I get them messed in my head, and usually tend to use Preterite, because it is simpler -- one word instead of two.
How? Break the Preterite on two parts: the Preterite 1 is to be introduced before but close to the Perfect 1 and to contain verbs commonly used in preterite; the Preterite 2 is to be dedicated to the formal speech and to be introduced much lower in the tree (not sure where exactly, maybe somewhere around or even after Perfect 2?)
What do you think?