One of the maxims that I would repeat to myself back when I was a tyro designer was this: There is no such thing as a normal scenario.
The idea being that when you’re designing a scenario-based game, every scenario should have some kind of gimmick: this is the one where one of the commanders might defect, this is the one with the convoy passing through the gauntlet, this is the one with the exploding wagon.
You would think it would be easier to come up with these when you're doing a fictional or hypothetical game, but for me at least the opposite is true; I struggle when I'm faced with a blank page. I think the reason is that when I'm working from the history, I don't need to come up with what the thing is, only with how: how do I represent this thing in the context of the game's rules, and does that create an interesting and memorable dynamic?
Whereas if I'm making something up, I need to not only come up with the how but also the what. Often the two things pop into my head simultaneously - the what suggests the how or the how the what - but they lack that certain spark: the what suggests an easy how, or the how an easy what. Whereas with the history, I often have to work a little (or a lot), and as a result the how is usually something very striking and clever and it results in an interesting game.
The danger of course is to simulate too many whats with too many hows, resulting in a scenario that's not only baroque but also baroquen. I've seen a handful of fan-made scenarios for Table Battles for example that have all sorts of exceptions, all sorts of interesting gimmicks - and any one or two of them would make for a cracking scenario, but there's so many of them that none of them stand out. The scenario isn't the one with this or the one with that - too much noise, not enough signal.
There is no such thing as a normal scenario, but some scenarios are closer to normal than others. An introductory scenario is going to be more vanilla, because the point of the thing after all is to let the players splash around in the ruleset and get their feet wet. But you do need to give them something to splash around in: the worst introductory scenarios are the ones that do an enviable job of sticking to the core rules but forget to show the players why they'd want to play the darn thing in the first place. An introductory scenario might not have all the special bells and whistles, and might have fewer units of less esoteric types, but there has to be something there.