I think the problem with trying to make a Paragon choice the worse decision is it would be hard to make it not feel cheap. I mean, here's the basic scenarios I can think of:
a) Paragon takes a risk, it backfires. Unless this risk was clearly shown to be very, very, stupid and you were warned more than once not to take it, then it would just feel like a 'gotcha!' moment where you were being punished for picking the 'good' option.

Paragon chooses to save innocents but lets the bad guy get away. This has already been done with Balak in BDTS (and kinda in the Zaeed mission too) and I thought it was a great decision, but it's a rather conditional scenario and would feel forced if you kept running into it.
c) Paragon sticks to their principles even if the end result is worse because of it. I'd actually like to see this one. For once let the pragmatic or colder decision be the one that will make the galaxy a better place, though at a price not everyone is willing to pay or carry out. You couldn't keep repeating this though or it would get pretty dreary and irritating.
That's all I can think of at this hour, ultimately though I think Paragon
has to have good results and rewarding endings, for at least the majority of the time. If you didn't and playing Paragon just ended up with sucky results and risks backfiring all the time, then there'd be no reason to play Paragon if your principles aren't helping anyone, while in the current morality system Renegade does have good reasons to play (advancing humanity further, saving the many over the few, funny lines + coolness factor). You would also end up playing a guessing game trying to figure out if this Paragon risk will work out or backfire this time, and I don't think that'd be particularly fun.
Hope that made sense, my two cents.