I'm not trying to say this card is amazing, but since Eidolon has never been good for me, I'll test it out over that - I've been looking for an excuse to cut Eidolon for some time either way.
Doom Whisperer is an amazing card and has been a must answer permanent in my environment since you can't chump it. Death Tyrant however can also not favourably be chumped and reads like a very interesting card. I think Doom Whisperer is a faster clock in most boardstates, so I'd say it has an edge there.
While reading wtwlf123's writeup it occured to me that my perfect cut for this card will be Eidolon of the Great Revel. That card just never did enough for me and with its RR manacost, it often feels like it belongs in exactly one deck. While Cemetery Gatekeeper is probably also relegated to aggro only, it has a better manacost and first strike.
Maybe this is because I'm playing unpowered, but the 3 CMC clause also just didn't trigger as often as you would see it in Legacy where decks are super lean, so I'm happy to see Eidolon go and have a replacement.
After thinking about it for a while, I think I will cut Fact or Fiction for this. Hiding information and essentially making a 2 card pile for yourself seems better to me than digging one deeper. I think however it all depends on this crucial question on whether 4 is much worse than 5.
Cuts are hard, especially for pure value spells and not archetype defining things. I really like Chart a Course, so my cut will either be Search for Azcanta, Impulse or a 3 CMC card I can't decide on yet.
I like this card as well. While 4 CMC in Black is really tough, I'm happy the options get more diverse!
On notions of powerlevel, I think you already mentioned the most important thing: All the cards this compares to fail in the face of instant speed removal and die to Bolt.
I think Foretell on Behold the Multiverse is way better than people give it credit for here. That card actually has the potential of saving you from Manascrew and fits neatly into curves or double-spelling turns
It ticks quite a few boxes to be honest, Evasion and Haste being paramount for red 5 CMC in my opinion. I'd say almost guaranteed value. Definetly worth testing out!
Maybe it's weird, but I like Altar of Dementia also as a regular card in control mirrors. Reshuffle effects are sparse in my Cube, essentially only Echo of Eons, Time Spiral and the Eldrazi Titans. Decking someone has been a real wincondition and with 40 cards it is easy to put your control opponent on a 5 turn clock or so.
I've been playing this card for some time now, having included it when I introduced Persist-Combo and Dark Depths to my Cube. All the reservations against this card are valid, but I just REALLY like its interactions.
Incidentally, this is also one of those cards that keeps getting better as more relevant effects keep getting printed, as for example Urza's Saga you mentioned.
That will probably never change the fact that this card does nothing at all on its own, but as an enabler I like it.
In that case, I'd say that as long as you carefully monitor the viability of Aggro, your idea has the merit of drastically increasing the number of synergistic 2-drops. On the other hand, these synergistic 2-drops will then also be prioritized not only by the archetype drafter but also by the Aggro drafter.
In any case, I'm interested where your experiments lead!
Maybe this is because I'm playing unpowered, but the 3 CMC clause also just didn't trigger as often as you would see it in Legacy where decks are super lean, so I'm happy to see Eidolon go and have a replacement.
I really like the Lifegain on Fumigate, and Doomskar is occasionally online very fast.
On notions of powerlevel, I think you already mentioned the most important thing: All the cards this compares to fail in the face of instant speed removal and die to Bolt.
Incidentally, this is also one of those cards that keeps getting better as more relevant effects keep getting printed, as for example Urza's Saga you mentioned.
That will probably never change the fact that this card does nothing at all on its own, but as an enabler I like it.
In that case, I'd say that as long as you carefully monitor the viability of Aggro, your idea has the merit of drastically increasing the number of synergistic 2-drops. On the other hand, these synergistic 2-drops will then also be prioritized not only by the archetype drafter but also by the Aggro drafter.
In any case, I'm interested where your experiments lead!