I remember hearing from my friend that he enjoyed playing as Hamburg for like first 100 years without grabbing any other province by just developing and abusing the fact game continuously considered nearby OPMs to be his only valid rivals for constant Show Strength wars. Whole game however, I doubt would be any fun.
I don't think there are any achievements that require you to be an OPM... It is really not a game made to play tall, not enough mechanics to keep things interesting.
But.. Maybe remaining an OPM and conquer only by vassalizing could be fun?...
So... Are we talking about literally only an OPM, without vassals, colonies or TCs?
Even as a heavily, heavily inclined tall player myself, I can't possibly see the point of doing that.
Even that would be hard to do as a pure OPM.Just focus on pointless money.
I don't think there are any achievements that require you to be an OPM... It is really not a game made to play tall, not enough mechanics to keep things interesting.
But.. Maybe remaining an OPM and conquer only by vassalizing could be fun?...
Don't know if it counts but I'm playing a game as Hamburg where I've stayed as an OPM with no allies, but I do have vassals. 14 of them in fact, which puts me way over my relation limit, but it works. It's pretty interesting since vassals give land force limit but the manpower doesn't change, so I have a force limit of like 120k but my max manpower is only 30k or something. I also decided to dismantle the HRE but I think it would've been better for me to stay as a free city. Anyway it's been an entertaining run, I basically have entire northern Germany plus half the Netherlands vassalized, all loyal, as a 63 development OPM.
I've done multiple runs from 1444 to 1821 staying as a OPM. For me, the fun comes from the challenge of surviving and the need to adopt strategies that are different from playing wide; it adds variety to my gameplay. Hamburg and Frankfurt (or any free city, really) are fairly easy choices and make for a relaxing game. Lubeck is a bit harder since you are not a free city and, therefore, are not protected by the emperor; however, it is more interesting since you can focus on expanding and managing your trade league. Surviving as The Knights or Riga is difficult. Making a custom OPM is also fun.
To flip the premise, do you not get bored snowballing every game? What I mean is, when playing wide and not arbitrarily limiting yourself, you always reach a point where you are practically unstoppable. A constant complaint on this forum is that the game gets boring around 1600 because you're unstoppable by this point, but then players complain anytime the devs introduce mechanics to limit expansion and conquest (admittedly, these mechanics usually just make expansion more tedious rather than more difficult or strategic). I'm not trying to attack anyone here: I've just genuinely never understood this dichotomy.