Re: Is it a good idea ...?
"redesigning a taller undercarriage to accommodate the new, bigger engines in their proper place would be awfully expensive as well as adding weight, which reduces payload capacity."
Not at all. Such an aircraft already existed. It was called the Boeing 757 and works extremely well, as well as having greater payload capacity because it has engines with much BIGGER fans than can possibly fit under a B737's wings and are much more efficient.
The problem was, it isn't a Boeing 737 and pilots need 757 cockpit certification, ground handling crew need 757 certification, service crew need 757 certification, gates need resetting to different heights etc.