That was a pleasant surprise, the first time I ran an impromptu pick-up game, I found monsters to re-skin as elementals, and had 4 encounters ready in minutes.Reskinning things was so easy and fun.
That, and a playtester comment about the warlord, really set the tone for me.
I just ran a complete imorov, that way, tonight. An absent player unexpectedly got to return, and we threw together both monsters and alternate PCs, to play through the situation he'd been left in.You didn't even NEED a MM. Just grab some of the various charts with the math and have at it. I ran complete improv games that way in 4E.
But I'll do that in any ed, in 4e, just with stats... that I don't have to fudge constantly...