I wish you could retrain mercs or fold them into identical factional troops.
I THINK, if the merc unit has the same skin as the factional unit, that might be possble. At least in 0.80 I've been able to retrain Thessalian Cav, acquired as mercs, in the cities, while I coudn't do that for the hoplitai, and in both cases IIRC the merc and factional units shared the same skin.