As an alternate take on Fervor of the Converted, but making use of stealing opponents' Knightmares as well. This name matches that flavour a bit less well, so I could change the name a bit. Signpost for the (Blue-red archetype).

Taking a cue from Improbable Alliance, this gives you an expensive way to do its thing built in, but works much better if you find other ways to do it.

I get why you chose to exchange control of lands, but lands are the likeliest permanent type to have ownership overlooked, especially with a lot of changing going on.

No longer piling all my Mountains because two are mine, two opponent A's and one opponent B's is just the kind of hassle that I'd rather avoid in an uncommon design.

It's only until end of turn... was my thinking, at least. And assuming the opponent taps their land in response, you don't need to treat it like a land, really; just a miscellaneous permanent sitting waiting to give back at EOT.

But it is a bit "do-nothing". It's not as useful as Improbable Alliance's loot. I'm not a huge fan of this design, for that reason.

Hm. Maybe that's something else that might be useful but isn't just gain control, eg swapping tapped permanents?

I definitely missed the "until end of turn". I just mentally occluded that part for the reason you stated. It just seems too particular to the card. Hmmm...

"Exchange two tapped permanents UEOT" (or two tapped nonland permanents) seems interesting. It'll let you steal static abilities or other fun stuff like that. I don't love the way it lets you remove an attacking creature from combat though. ": Fog target non-vigilant attacker" could get annoying on an enchantment.

Aha. That's the way to do it. Remove the silly do-nothing activated exchange and give this a powerful one-off exchange instead (taking over the role of Hostage Exchange).