" The first, in Latin; which we published lately, called the Formula Missae..."I find this an interesting mix of 'Divine Services'. Type 1 and 2 would focus on outreach to the general public.
" Next, there is the German Mass and Divine Service, of which we are now treating. This ought to be set up for the sake of the simple laymen. Both these kinds of Service then we must have held and publicly celebrated in church for the people in general. They are not yet believers or Christians. But the greater part stand there and gape, simply to see something new: and it is just as if we held Divine Service in an open square or field amongst Turks or heathen. So far it is no question yet of a regularly fixed assembly wherein to train Christians according to the Gospel: but rather of a public allurement to faith and Christianity"
" But the third sort [of Divine Service], which the true type of Evangelical Order should embrace, must not be celebrated so publicly in the square amongst all and sundry. Those, however, who are desirous of being Christians in earnest, and are ready to profess the Gospel with hand and mouth, should register their names and assemble by themselves in some house to pray, to read, to baptize and to receive the sacrament and practise other Christian works."
(The German Mass and Order of Divine Service, January 1526. http://history.hanover.edu/texts/luthserv.html)
I suspect Luther recognized that the large gatherings was not the best fit for continued spiritual growth. I suspect he was looking at what was modeled by the the New Testament gatherings as he considered a home based gathering of believers.
I find it especially interesting that he proposed having baptism and the sacraments practiced in these smaller house gatherings.