A threshold would make the most sense if there is a seam that has any movement. That seam will transfer to the flooring and will be quite visible (assuming you're using a vinyl flooring of some description). As for the small rotten bit, once the leak is repaired, you could cut that bit of rotten plywood out and scab in a new piece. Fill any seams with a non-shrink wood filler.
Cameron & the Labradors, Kai & Samm
North Vancouver, BC
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