Filter out outliers of the scRNA-seq (heterogenous cells)
From what I known, there is no clear consensus in the field and it depends on the type of cells you are interrogating. 

However, if mitoRNA/endogenousRNA ratio does not fit for your purposes, other option is to check at the total number of genes/transcripts detected in each cell. In this way, you can filter out cells that have considerable less genes/transcripts detected than the rest since this may be indicative of bad quality cells for any reason (apoptosis, RNA degradation, sequencing itself, etc). For instance, to put a threshold of >1000 genes/transcripts in a cell to be considered for further analysis.

Consider also to take a look at the number of mapped reads for each cell, as cells with low mapped reads respect to the rest could be potentially problematic.