I really like those ideas as well. But you are right that some of them are a bit unbalanced. For example with the slime class, not being able to experiment on them in the laboratory will pretty much make sure that they cannot gain those 2 bonus points that any other race can get for both strength and agility by using said laboratory. Would that not be a little bit harsh? Maybe instead give them a -2 to magic instead.
As for the thief class that you mentioned, while it is a great idea it also is extra money without a penalty for it. If we would expand upon it a bit then a good thief would not get caught but after a while people would still realise that they are starting to miss money, and eventually would connect the dots, with you being close every time some of their money goes missing.
Would a small loss of reputation at the end of every week in the city where your mansion is located be a sufficient counter penalty for your thief class?
EDIT: Also another small idea, but i truly do not know if this is possible. But maybe set the players stats by race, instead of giving the player the ability to pick their own stats. (What i mean by that is normally after the player picks race they can decide how to divide their stats, with my suggestion your stats would be dependant on what race you pick. For example, the player picks a demon as race instead of getting to decide yous status after that have it literally have the status of an ingame demon.) Namely at the max without improvements 5 to 4 to 4 to 3 in strength, agility, magic and endurance respectively.
With high regards,
Reinyn