I never found the way Slaanesh's tempation works currently, steal enemy units for money, to be all that thematic. It's also yet another mechanic that has no interactivity and that's honestly a design flaw. So here's my suggestion for how to change it so it better represents what Slaanesh is all about.

Instead of stealing a unit, Slaanesh can offer to assign meaningful buffs to up to three enemy units (starting with one and having to research for #2 and #3). For every buff the enemy accepts, Slaanesh's army abilities accumulate points faster and come off cooldown quicker, plus Slaanesh gets an after battle bonus for defeating this army (more XP, better items, more casualty replenishment, something like that). If the temptation is rebuffed, the units that were tempted suffer 5-15% damage instead but this damage can never be lethal. Only mortal units with an independent personality can be tempted.

I find this would be a more interesting interaction than what we have right now.