How about (ni) supozu ke?
I’m going to take a different route than other responders. Searching La Tekstaro, it appears to me that changing words — from imagu to supozu — may idiomatically be the best choice to capture the connotations of “imagine if” you posit.
First, I just searched for imagu vs. supozu. Two things jump out immediately from the results:
- imagu is used mostly in direct quotation, while supozu is more-often used in the main text.
- imagu does, as your question points out, take either se or ke, with different connotations. But supozu seems to, almost always, use ke.
Then I compared two more-targeted searches. (If you don’t know regular expressions, I explain them below this answer.)
First, imagu\W?\s+[sk]e. Indeed, it appears frequently in direct quotations and only a few times in the first 100 results in the main text. As said, the usage of se and ke varies.
Finally, ne imagu occurs rather frequently—without always having the idiomatic meaning “don’t imagine…” has in English of being a mild or veiled threat (e.g., “don’t imagine you can continue to displease me”)—frequently, it is literally used as a negative imperative phrase.
On the other hand, a search for supozu\W?\s+[sk]e, returns ke vastly more often than se (you won’t even find supozu se in the linked search, you’ll have to expand the parameters to find it). Also, a large proportion of the results are in the main prose.
If you spot-check the results (you can tap the underlined matches to get a full paragraph’s context for each), it seems like the connotations of supozu ke is more along the lines of what you’re getting at—introducing and positing a hypothetical future for further consideration, rather than exhorting an expansion of the interlocutor’s mind (like, say, in John Lennon’s “Imagine”).
So while this isn’t a case of “false friends”, it does seem to me to be a case where the cognate isn’t the best choice since imagu supports much finer grains of subtlety. Supozu ke, however, seems to nearly always mean basically what you’re going for.
The one drawback, I think, is that if you’re talking about something like winning the lottery, Supozu! as an intensifying interjection doesn’t really work as Imagu! does.
I’d also note that the indirect first-person imperative ni supozu ke… (“let’s suppose that…”) seems to be as common, or nearly, as the second-person imperative supozu ke. As a main-prose introducer, both forms are semantically near-identical, as they are in English:
- Suppose that the moon were made of green cheese…
- Let’s suppose the moon were made of green cheese…
Writers differ in Esperanto, just as in English, on whether to use the first-person plural (“we” or ni) or the second person (“you” or vi, but usually—in both languages—just unstated) in such contexts.
They generally read the same, though I think in educational contexts (textbooks, explanatory essays, and so on) the first-person plural “let’s” is more frequently encountered, since the writer is trying to imply an active involvement by the reader. In La Tekstaro there seem to be many more attestations of ni supozu in early Esperanto history and more of (vi) supozu in more recent Esperanto.
p.s.: Regexes
The regular expressions imagu\W?\s+[sk]e and supozu\W?\s+[sk]e say this:
imagu: Search for imagu or supozu. (Technically, this should probably be \bimagu and \bsupozu—beginning with a word boundary \b—to avoid cases where these are just the ends of longer words, but it didn’t matter in this case.)\W?: Once you’ve found that, look for a possible single non-word character—this is to allow for the possibility of a comma or other punctuation mark.\s+: Then look for some space so you separate imagu or supozu or imagu,, etc., from the following word.[sk]: Look for either an s or a k.e: Finally, look for an e to make se/ke. (Again, it would technically be more correct to end with another \b to avoid cases where se or ke begin a word, but again, it doesn’t really matter here.)
