Apologies if either idea has already been done - considering the source material, it's probably quite likely...
I've been here just over a month, and thought I'd make a contribution other than comments. As I'm sure I don't posess the literary acumen to pen an entire story (even a short one!), here are a couple of plot seeds to mull over (i.e. not complete detailed plots, but nuggets around which a plot could be built, around which a story could be built). Feel free to use / abuse them as you see fit. You don't have to credit me, but if you do create a story using one of the ideas, a quick DM along the lines of "You might like to read this... :)" would be appreciated :)

1) Pygmalion.

I'm sure you're all familiar with the George Bernard Shaw play (or at least, a certain musical adaption entitled "My Fair Lady"). Similar concept to the original, but obviously the big difference is that the target is an ordinary bloke, who's presumably danged a significant sum of money as a carrot to go ahead and learn to be a convincing female in some form of very public event (society dinner/ball?)

The emphasis will be on the fact training takes time - not the "dress him up, give him an afternoon's practice - almost instant pass!" cliché (which works well for stories where the emphasis is on how the protagonist copes after transformation / learning on the job [so to speak]) - the idea with this one is that by the time the event comes up, he's ready in the majority of frequently used female mannerisms as well as appearance. And is hopefully smart enough to make a firm association between the clothes and mannerisms, so he doesn't slip up either when operating in "guy mode" or "girl mode".

I suppose the easiest setting would be a US High School (Homecoming ball / prom), but, although more challenging, it might be possible to set it in a workplace somewhere. Taking on a 'student' full time (as with Eliza) would be a bit suspicious (especially if set in the present), so working on the 'training' during evenings / weekends whilst the protagonist carries on male life as normal during the daytime might be more convincing.
If you fancy an additional challenge, how about a short haired protagonist, who whilst relatively slim (for a Male), might have big feet (so online/specialist footwear retailers rather than high street) - the idea being the transformation into convincing female would be difficult (how about 6-9 months to effect the complete training schedule?) but not impossible. It could be fun to end the story just after the ball/prom/event etc. and throw open the gates to alternative continuations of the storyline...

Needless to say, in case it isn't already obvious, humans only - no talking animals, aliens or magic. Transformation must be entirely due to the collaboration between the person who made the bet, the protagonist, and a limited group of people involved in the process.

2) Fairy tales.

Rewrite a classic fairy tale with a TG twist. The obvious example would be Cinderella - perhaps the fairy godmother/father's spellmaking isn't what it used to be...(well, you've got to find some excuse for turning a boy into a princess - preferably a bit more inventive than "closet TG"!)...and how would you handle "...if the shoe fits..."?

Unlike Pygmalion; magic, fairies, talking creatures etc. are perfectly permissable (although extra-terrestial intelligences would be pushing the boundaries of the genre a bit too far for my liking!)

3) Venus Cursed - Doctor Who spin-off.

Well, it's already been done for "Gaby" and "Bike" - but whereas in those occasions, the Doctor turned up in the plot and surprised the characters, a Who adventure with the "Venus Cursed" characters could conceivably turn the tables and open a portal into the TARDIS, thereby surprising the Doctor :)
After all, if they ever opened portals to places in their universe's history rather than fictional universes, they could find themselves messing with "wibbly wobbly timey wimey" stuff and possibly wonder at the potential implications of the "Grandfather paradox".