After the introduction of the awakening I think that the new girls (not those already in harem) should start from a defined level, without using books for upgrade, instead of level 1.
To balance it out, I thought that half the level of the player was fairy for everyone (new and old players).
Example: for a player at lv 320 when he takes a new girl this will be at level 160, for a player level 100 the new girls will be at lv 50.
After lv 250 all girl need gems so this would help at least to grow new girls.