I know this is an old question, but recent developments in astronomy have caused this to become relevant again. Early in 2016 there was a paper published by Konstantin Batygin and Michael E. Brown (here) which indicates the possible existence of a new planet, dubbed "Planet 9". Subsequent work by other authors have lent more evidence to its possible existence, e.g., a paper by Fienga et al. (here) which concludes that including Planet 9 in orbital dynamic models decreases residuals.
This being said, the original paper specifically states
...our calculations suggest that a perturber on a $a'\sim700\:AU$, $e'\sim0.6$ orbit would have to be somewhat more massive (e.g. a factor of a few) than $m'=10\:M_\oplus$ to produce the desired effect.
Such a planet is certainly massive enough to have cleared out its orbit under normal circumstances. For reference, Neptune has a mass of $17\:M_\oplus$ while Uranus has a mass of $14.5\:M_\oplus$. The special feature of this planet is that it is located $700\:AU$ distant from the Sun which is in a very ambiguous and unknown region of our solar system. It is not clear at all how much, if any, debris in the form of comets resides out there. There are models suggesting a disk (referred to as the Hills Cloud) exists at those distances, implying that this new planet could reside inside an extended cometary disk. Of course this is only speculation, but it is a distinct possibility.
If the above-mentioned disk exists, we have to consider a few scenarios.
- Planet 9 was "inserted" into this disk by being expelled from the inner solar system early on in its formation. This is a possible cause for its current position and suggests one of two outcomes.
- It has existed here long enough that it has cleared out its orbit, thus making its planetary discriminant well within the "planetary" side of the definition.
- It has either not existed in the disk long enough to clear the orbit, or has achieved some sort of equilibrium such that it can't now clear the orbit (considering most of the clearing occurs during formation). Here is where you may run into an ambiguous case. For all intents and purposes we may consider this a planet, but there remains the possibility that, despite it being more massive than the Earth, it could formally be considered a dwarf planet or else exist in this fuzzy region as you suggest.
- Planet 9 formed at its current orbit (plus or minus some migration).
In this scenario I think it is clear that it would have cleared its
orbit and resoundingly be considered a true planet.
Now these are all wild suppositions. I don't have any of the math to back it up, and I'm not sure the numbers to plug into the various equations are well known, or even exist. I'm merely remarking on the possibility that the situation you asked about now seems like it could exist.