First of all, yes, some achievements simply have to be challenging. Here's simple example from my experience: I've achieved "real deal" in L4D2 in SP (with 3 dumb bots on my side) - after over 60 deaths. And when I told this to one "experienced" L4D player - he didn't get it. "Oh, pal, you've never really played L4D" - thought I : ) I mean, some achievements really should differ good experienced players from casuals who play the game for second or third time ever. Not all, but few extra-pro))
And I glad no additional entity will be needed for circuits - or else it would be quite reasonable usability limiting factor.
Simple wish about fire spreading - some limiting factors should also be. The most obvious seems to be rain system, however this brings up some technical issues - e.g. it won't be logical to get rain on whole map simultaneously, including deserts etc. Making rain clouds? It will probably need intending some meteorology math in game, thus making this rain idea not so cool (unless YOU, guys, see cool and "cheap" way to intend it, which I believe is possible for you)
Another way I think is more natural fire spreading - different fire catch chance for different tree types. Say, dried dead tree will flame from a tiny fire spark, but healthy tree in wet forest will need few fire sources nearby or longer flamethrower beam focus, thus preventing single flamethrower (possibly accidental) click burning square kilometers of forests. Well, chance still should be - just like IRL.
Holding formation further and further,
Millions of lamb stay in embrace of Judas.
They just need some bread and faith in themselves,
BUT THE TSAR IS GIVEN TO THEM IN EXCHANGE!
Original: 5diez - "Ищу, теряя" (rus, 2013)