I just had an idea, while I was writing in the Harbinger topic.
Currently tainted (other than Harbinger) is strictly a setback. Neither of the rest of the cards interract with tainted in any way at all.
Since other than Merchant and Harbinger the other cards are relatively fine, how about breaking this stigma with Merchant? This could solve the current possible infinite combo issue too.
I thought of something like: (Maybe give it better stats, but if it would be strong enough with the new mechanic, maybe it is fine like this.)
When Merchant of Souls enters the field, gain X energy, where X is the ammount of life you lost this turn.
E(1) Play a non-spirit creature with level 2 or less from your graveyard using Merchant of Souls' energy to pay that card's mana cost. Use this power during your turn.
It would start from 0 energy, or maybe more depending how strong it would be.
If it would be too strong X could be halved rounded up/down. Or something like: it starts with for example 1 energy, and gets X halved rounded up energy when enters.
Maybe we should make it legendary, or maybe since it costs a lot, it is fine if they play 2 of it the same turn.
We probably don't want him bringing himself back, so:
-It should state non-Spirit non-"Merchant of Souls" (maybe give it a new subtype, like "Summoner", so non-Summoner).
Summoner type could be added to new creatures that can summon many things, and happen to look quite different from being spirit, and we wouldn't want them to be reincarnated over and over again with Merchant. Maybe Harbinger and Banshee should be a Summoner too, and let Merchant summon Spirits too (though that might still be a little problematic bringing back Mesmerizing Spirit or Disquisitive Spirit or ... Shifting Aurite ... non-Spirit stays lol

)
-Simply make him a spirit (though he doesn't really look like one).
-Make him 3 lv.