by Nikolai Teleshov. Mildly arhaic but easily understood by modern speakers:

- «на другой день» is not much used as "the next day" anymore
- "замыслить" is not likely to be used by a native in the meaning "conceived, though of some design", though "замысел" (intention, plan) is still in use.
- "..какой замыслила.." (the like she conceived) is generally not a phrase a native would use nowadays, especially mid-sentence.

замыслить - замышлять.
Нормальный глагол, всё в ходу.
Только не в том смысле, как здесь, а в смысле "замыслить что-либо плохое".
Хотя "замышлять" употребляется заметно чаще, и налёт старины на совершенном виде есть.
Но я бы не торопился с утверждением, что носитель его едва ли использует.
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I don't think it's as hardly used as you think, just not the way this sentence uses it, but in the speficic meaning 'to conceive, premeditate smth bad'. Though it does sound quite old-fashioned anyway even in that sense, unlike the imperfective "замышлять", which is fully in common use.

Fixed.
"Замышлять" is a different verb and is irrelevant here.

"Замышлять" is a imperfect form, "замыслить" is a perfect form.
"Замышлять" is more often used.

Depends on how lax you are in the definition of an aspectual pair. They are different verbs, anyway, even if you consider them a good pair, which is questionable. I think few here are curious enough to discuss theoretical views in this field.

On the other hand, some people even consider "искать" and "найти" a pair.

>some people even consider
Well, they are a pair, aren't they? Only a suppletive one (non-suppletive would be искать - отыскать).
True that in this case the two verbs [whether such pairs are made of two different verbs or two forms of the same verb is another question entirely...] are generally used much differently, but they still share the basic link. A word's meaning is more than its logical value, though, and they're clearly different in style. But as long as they have a common meaning and there is a position where they can be construed to mean the same thing bar aspect, they can be linked into a pair on the occasion. This doesn't negate that most of the time they don't work like a pair and the link is feeble enough.

"On the occasion" is exactly why they are rarely considered a model pair. One important property that most linguists consider essential for aspectual pairs is that imperfective can express repeated action of the perfective counterpart. Which is clearly false for "найти/искать": "Я каждый день искал золото" is never understood as "found gold every day". You should use "находил золото" instead.

That is, the linguists who believe in aspectual pairs at all (I am sure, there are some that don't)

"Он что-то замышлял" - несовершенный вид, "Он что-то замыслил" - совершенный вид. Так же как "Он что-то проверял" - "Он что-то проверил".