vj, mj, bj

In the majority of Slavic languages the sequences vj, mj and bj of Common Slavic became vl', ml' and bl'
But in some languages like the West Slavic branch and Bulgarian/Macedonian, they did not

Examples:
obnovjat vs. obnovl'at
zemja vs. zeml'a
prisposobjat vs. prisposobl'at

as you can see I had two examples of verbs that have a -at ending
so this would also affect the imperfective verbs made from perfective verbs

in terms of a naturalistic Slavic language the change of vj > vl' mj > ml' bj > bl' should happen because it happens in the majority of Slavic languages and because l' phoneme occurs in most Slavic languages