The discussion about treasure hunter made me curious on the thought process behind traveller effects. Looking at the secret history it seemed like a number of the cards picked were cards that you previously tried but didn't work out for various reasons and making them part of a traveller line seemed to fix those issues.
Is that sort of how it went, or did you have more specific criteria for the kinds of effects you wanted traveller cards to have? (e.g. something that synergizes/anti-synergizes with the other cards in that line, a niche card or generally useful one, something you might want to stop at in some instances, etc.)
Well for the step where I first had the two linear paths, I picked names first. One line was just someone getting better, the other told a little story. Then, the effects wanted to at least somewhat fit the names.
For the Page line, I liked the idea of having just one attack (and one in the other in the obvious Soldier slot). The names thus had to work with not being attacks despite being names that might otherwise go on attack cards. One thing was to have a Moat, but that had to go on the top, since the under-the-line space was reserved for upgrading. So, there's an attack, a Moat, and two treasure-gainers, which also fits with the names. Your Hero fights, but doesn't fight the other players.
For the Peasant line, things were more of a mystery. Soldier wanted to attack and Teacher wanted to hand out tokens. Fugitive and Disciple did not suggest as much in terms of what they would do. At one point the Fugitive went on the Tavern mat; that was cute, he hid out in the Tavern. That was part of trying to make Teacher harder to go nuts with, but was very wordy.
It was natural to look at old ideas to see if some of them would work; I mean I needed 8 extra cards. Fugitive in particular was a card I was resigned to never doing, but I could just do it here with no issues. Treasure Hunter you know about. It had been in Hinterlands, it was pretty cool there, what with Haggler and Border Village and so on. Disciple tried another classic dead card, but didn't keep it. Hero tried something new simple & exciting, that would be dangerous on a regular card. Champion was a Moat variant, then Moated from anywhere. Teacher was new. Soldier and Warrior were just very simple attacks using the tokens (at first). Attacks are hard and that was an easy way to get terse new ones.
There were things that weren't a reason why a card showed up, but which then seemed nice, and made it less likely that that card would leave. It was cute that Page and Treasure Hunter were +1 Action while Peasant and Soldier weren't. It was cute that Peasant was the +'s not on Page, but it didn't start that way. It was cute that Champion was a duration and Teacher a reserve; neither started that way. It was cute that Soldier gave the -1 card token and Warrior gave the -$1 token, while Soldier gave +$2 and Warrior gave +2 Cards.
I liked the idea of trying to have each step be worth stopping on sometimes, but it was clear you would usually want to push to the top with your first Page/Peasant. I do think it ended up where every step is worth stopping on sometimes, except Page (and yes even then but way less often). You don't stop on Fugitive that often, but I have done it. You stop on Soldier, Warrior, and Disciple all the time; and then sometimes there's a combo for Treasure Hunter or Hero (or you don't need Champion).
Are there types of effects you think would make a poor traveller upgrade but would work as a card? Vice-versa?
Mostly what makes for a poor upgrade but not a poor card, is something that's missing out by being an upgrade. You see the upgrade less often; it would bring more joy to the game as a regular card. There's also, things you need early game, since the upgrades are delayed. Arguably when an ability is precious (e.g. +buy, in some games) it's bad to have it on a traveller (other than the 1st or last one). I kind of shied away from those but not completely.
A card that's too narrow as a regular card can hope to find life in a less-used slot like the travellers; if this isn't the game for Treasure Hunter combos, you just play it once and move on. A card that's hard to cost (e.g. Fugitive) gets this new option of costing time.