This thread reminds me of the final fight scene from The Last Dragon. If there was ever an example of "leveling up" in the middle of combat, that was it.
I haven't seen the movie. Was it something like this: "Heroes try new things and fail, repeatedly, as they identify and mobilize against a threat, and as they come together as allies. In the Big Final Fight Scene, the heroes are on the ropes; it's time for the adversaries to smugly sneer and finish off the heroes. Then the heroes suddenly rally and successfully apply the lessons they've been learning all along, surprising and toppling their adversaries."
If I just now accurately described a movie which I haven't seen, then there's probably a name for this pattern on some website about tropes in TV storytelling. As a DM, I want an option to impose mid-fight-scene levelling on PCs, to set up a scene like that at my D&D table. The 5E RAW provides this option, by means of Rule Zero if not elsewhere.
Obviously any strict method of XP awarding and leveling is going to leave something to be desired on the logical-functional-fun matrix.
(tongue in cheek): I demand, for 5E, simulationist level gain mechanics, based on careful observation of how athletes and soldiers level up. Has a triathlete ever levelled up between one section of a race, and the next section? Consider the SAS agents who responded to the 1980 Iranian Embassy siege; did any of them level up during the operation? Or during the long rest immediately following that operation?
(end tongue in cheek)
Yeah, that's why I don't expect ever to see mechanics for the process of level gain which make sense to me, because level gain itself doesn't make sense to me. Most of my TRPG over the last 20 years used systems with more incremental, fine-grain resolution of characters gaining new abilities one at a time, starting with character generation. Those systems don't use levels or classes. My return to D&D is mostly fun, I enjoy 5E much more than 4E, but levels and classes feel like a big step backwards compared to HERO System, GURPs, White Wolf, etc.
The 80 gp could be spent preparing for a 40 day journey to the ancient oak grove, preparing a special gift for its unicorn guardian, and preparing special robes and a golden sickle needed to learn the final magic of unlimited shifting.
Okay, that works for me, thanks for a nice fill-in. I still intend to use the time and GP costs as a rough ballpark average, to be modified according to specifics; which, fortunately, RAW allows, by application of Rule Zero (to a 5E DMG rule presented as optional anyways).
I also figure this: most training is easiest and fastest if you have a trainer who already has mastery of the skillset you're learning. But if there's no trainer - perhaps if you're the first person to EVER develop the skillset in question, or the first in several generations - then there are ways for you to train yourself. Those methods might be tedious and time-consuming, they might involve more failures along the way, they might involve the assistance of a deity (or a paladin oath, a warlock patron, a Mary Sue NPC, etc.), they might require using up a lot of raw materials, they might require collaboration between PCs. But if you keep at it, then you'll break through, sooner or later.