Id like to see one free turn over of the slaves each day instead of the automatic one.
That to be able to stay with one really good slave for some time, until its room in your stable to take him in.
Or alternatively be able to buy slaves, and just keep them until you got room in your stable to train them. Maybe against some gold upkeep to pay for feeding?
Lastly id like to say i dont see any reason for keeping the stable limit to only 5 gladiators.