What types of Church reforms might have arisen out of the Northern Renaissance if the Reformation proper had not occurred?
It seems likely that doctrinal reforms might well have arisen from the Northern Renaissance had the Reformation not happened. However, it seems very unlikely that they would have gone very far in undoing the hierarchical nature of the Catholic Church.
Ideas like those of Erasmus could have led to reforms of the Church. It could have allowed and encouraged lay people to read the Bible for themselves in their own language. It could have led the Church to push its clergy to be less money-hungry and more attentive to the spiritual needs of their flocks. Centuries before, for example, the Cluniac reforms had shown that it was possible to push the Church away from wealth and more towards piety. These sorts of reforms could have been implemented without threatening the entrenched powers of the Church.
But it would have been very difficult to reduce the importance of the Church hierarchy. It is never easy to convince people (whether kings or popes) to give up significant amounts of power. Therefore, the reforms would have been much narrower in scope than Luther's.