Mandrakes should be a unique and versatile unit. However but are rarely played simply because they carry little punch at the end of their nifty deployment tricks. Rather than giving them new skills or a leader their specialty should be enhanced to emphasize their character and improve them. I would make the following modifications to the Hidden Deployment rule: Rather than placing 3 models on the table to represent the location of the unit the DE player should be able to place all the models in the squad across the table as infiltrators. This will represent the possible lurking locations of the Mandrakes and create terror and confusion much as they should. Mandrakes may remain 'lurking' through to the last turn of the game. While Lurking they may move as described in the Codex. Mandrakes should never be scoring units.
Grotesques should be a more versatile choice. An army should be able to rely heavily on these units. They should be considered heavy choices. A variety of changes should be available. Regular Grotesques should be able to be taken in 20 unit squads provided they are lead by an Homunculi. Uber Grotesques: up to 10 unit size (as detailed in current codex). Sabotage Grotesques: up to 10 unit size. Hidden in a single 10 man DE Warrior squad. They act as a warrior squad until revealed in the movement phase and they become regular grotesques.