[arcanus]

About 50/50 within our campaign, alt/real world historical scenarios can be just as challenging and horrific, one of my groups most challenging and harrowing worlds was rescuing an army officer in the final days of the Fall of Berlin.

A race against time to stay ahead of the Red Army war machine whilst dodging a fight to the bitter end Wehrmacht.

Even from an action perspective, sensible use of enemy numbers, ammunition etc means that even high powered PCs are under threat in echoes or normal parallels.

Opponents are not stupid and nor are they all cowardly, a PC that dosen't go down from a volley of fire that should put them down is just as likely to provoke overkill rather than retreat - fight or flight, my group are aware of that so dependant on their Ads/Dis's play accordingly, Overconfidence can be tricky at times.

As previous posters have said normal echoes or parallels also present opportunities for intrigue, espionage and detective work which my group particularly enjoy.

One of the big ingredients is interesting concepts or memorable npcs

For me supernatural or superscience just creates alternative mystery, it allows the PCs to use their more unusual abilities slightly more liberally or create a fantastical story whilst letting me outlet my imagination differently.