Well, IMO the Hoi3 WW1 mod worked quite fine. There aren't really that many alternate history possibilities beyond Russia, at least not too many more than for the vanilla game I would think.
well, truth is that here are MANY posibilities, even outside of russia. especialy after WWI ended.
what if west decide to killl "commie beast" in craddle and decide for full intervention until reds are defeated ?
what if reds decide to support more commies revolt in hungary, so they will be ale to resist romanian invasion and crush of thier revolution ?
how much thier resistence create similiar revolts in other countries ? especialy germany.
what if czechs decide to try for peceful solution regarding silesia area instead of short victorius war, which plagued future relationship ? (but make penalties strong enough until dispute is settled, so it make sense to solve it fast, even by limited war)
what if soviet-polish war ended diferently ? like reds conquring warsaw or poles able to keep ukraine ?
and i agree with you, wwI era, and posibility to play in gap 1918-36, and well even 14-18 will be very nice, and i will love to have that dlc.