I feel like some of the current urban legends can fit well with some other characters. For example, Aya goes well with the Turbo Granny one, since speed is her thing. Sekibanki goes well with the red and blue toilet papers one, since that one involves head-related fatalities, and she's all about "losing her head." Her cape is even red and blue. Hasshaku can go along with Sanae, since it involves buses, and being from the outside world... Yeah...
Don't get why Ichirin's UL is Hasshaku and also Marisa's UL, though...
It fits the characters in Urban Legend in Limbo.
You can't argue that Byakuren doesn't fit Turbo Granny.
You also can't argue that Sekibanki fits better than Miko, since Red/Blue is Miko's thing in Hopeless Masquerade. There are other characters with red/blue deal, like Chen, but Miko is previously playable and is the most obvious.
Marisa is the one with a somewhat random urban legend, which fits Sanae more, but Marisa is relevant to the story.
Some like Byakuren becoming a mouse, Koishi becoming a cat, and Futo becoming a chicken are pretty random.
Futo being a chicken is not all that random. She has a thing with overreacting with her fears. She burns temples to the ground because she fears them.
Yes, I know high quality sprites are harder and more time consuming to make than 3D models, but it's not like its impossible to create a lot if one is given a lot of time. There is no golden rule saying that one can only create a limited amount of new characters per game for sequels using the same engine after all.
And what makes you think time is something the creators have?
In either case, the legends themselves make as much sense for these Touhou characters as for other Touhou characters. It isn't like the other Touhou characters somehow have a stake in the urban legends.
It is also ridiculous to believe the characters wouldn't be re-used. That doesn't make any sense.