Past va'adim have been based on Alei Shur by R' Shlomo Wolbe, but the hebrew is not easy, and aside from small excerpts, we do not have reshut from his family to translate the sefer (and we have asked).
An explanation of our approach is in the translated excerpts of Ali Shur, but to very briefly summarize, the primary criterion for this list is that the book has practical exercises to be performed over a multi-week period to practice and develop specific middot, before moving onto the next exercise in the same middah.
Other seforim that have been suggested include the following. They almost all involve a bit of a change in hashkafic focus (mostly to be more deveikut-oriented) or methodological focus. Our real challenge here is to choose something that's translated/translatable that has the amount of middot development that we're looking for, so some of these books are more on target, and some are farther off.
- Bilvavi Mishkan Evneh in both hebrew and English.
- Sha'arei Orah by R' Avidor Miller (though some are concerned that his machshava approach (in other seforim) is offputting for some members of AishDas leadership. (Hebrew only, as far as I know).
- Seforim by the Piasetzener, namely B'nai Machshava Tovah (online in English HTML and in English PDF), and Chovat haTalmidim (for purchase in English). B'nai Machshava Tovah isn't really a mussar sefer since it's almost entirely focused on deveikut, but it does use a similar format to our va'adim.
- BiMesila Na'aleh which includes va'adim on Mesilat Yesharim (Hebrew only).
I'll try to keep this list updated if other suggestions appear on the AishDas lists.