The slogan of the 2020 Olympics in Japan is
未来{あした} をつかもう (Discover tomorrow)
Why is it read as あした？ I did not find a standardised dictionary with that reading, however I found あす as a reading in a dictionary for names. It seems it would be the same if it were 明日, but I still don't understand.
If it's for an aesthetic reason, does that mean I could put 'beautiful' readings onto Kanji I like? (Of course, only to a degree that is)
明日 can be read as either あす or あした. あした is more common in speech, while あす is more common in prose (like poetry) and formal documents, as well as older texts.
未来 is normally read as みらい (音読み) of course, and occasionally みく (訓読み). Poetically, it's also あす, as you point out, but this is uncommon. As in English, "tomorrow" is being used to mean "the future", as in phrases like "the world of tomorrow".
In this case, あした is applied more stylistically than anything else, and if there weren't the furigana above the characters I doubt anybody would read it that way. It's not uncommon for kanji to be given one-off stylistic readings that are non-standard. For example:
とある魔術の禁書目録{インデックス}, a popular anime and manga
泥棒{あなた}は信用出来ない (from the question linked below)
Why do some kanji have furigana that are not valid readings? discusses a very similar situation.
In this case too, reading 未来 as あした is stylistic rather than standard, used for artistic and advertising effect. Plus, it stands out as memorable as all slogans should. It's suggesting "world of tomorrow" (i.e. the future) in the poetic sense, just as is seen in other languages like French and English (just not in common speech because it sounds too ridiculous).
明日 can be read あす or あした.
あした is used in conversation, while あす is used in writing. But natives don't distinguish them.
In recent popularity, in order to emphasize the word we intentionally read words not in the correct pronunciation that the writing usually demands.
E.g. 本気 is correctly read as ほんき, but we see 本気{マジ} with マジ assigned as its reading sometimes.
So for no special reason, we read 未来 (future) as あした (tomorrow); this implies a bright future. This reading contains the meaning that it is not the far future but the near (nearness implied by 'tomorrow') bright future that we can reach.
