I've been replaying this game recently - for the first time since 2015 - and figured I might as well get around doing the DLC (which I had never played before) as well. Somehow, the DLC manages to be better - and more challenging - than the base game on a pound-by-pound basis, despite the fact that the base game was already amazing. In that regard, it was definitely for the best that I started a fresh playthrough, as that gave me the chance of comparing them rather closely.
The boss design was probably the biggest improvement over the base game, and you can see the transition from base Bloodborne and DS1/DS2 to some of the better-designed bosses in DS3 (especially the DLC bosses like Friede and Gael) in The Old Hunters. Except, I feel TOH did a better job of keeping the multiple phase/HP bloat/infinity combos design in check, which lead to a better - if perhaps somewhat less challenging - experience overall.
In particular, the Ludwig boss fight was one of the best I've ever fought in a Souls-like. Almost everything about that fight was brilliant. The "monster" phase was suitably challenging without feeling ridiculous or cheesy (like a lot of the tougher "large monster" Souls fights often do), largely thanks to the fact that it relies on Ludwig's extensive and varied moveset rather than on the convolutedness and size of his model (though obviously, like in all similar fights, this is a factor). Then, in one of the most poignant moments in gaming history, Ludwig regains his humanity mid-fight (a masterful example of ludo-narrative consonance that enhances the whole phase design), switching to a second phase that plays completely differently from the first while retaining the overall quality in moveset design in an - admittedly easier but still tough - humanoid form. Ludwig's visual design was also interesting. Gruesome, to be sure, but befitting of both the lore and the overall thematic flow of the campaign. Not to mention the fact that that the Rapehorse is immensely memeable and a change of pace from the typical wolf/ape beasts that the game saturates you with.
Lady Maria was also a great fight. As with Ludwig, there is an adroit weaving together of lore, themes, art direction and gameplay that ends up being ridiculously climatic. However, it is also a much easier fight not only than Ludwig and Orphan of Kos, but her DS3 counterpart Friede (still harder than most base game bosses - including the similar Gehrman - and respectably difficult in its own right). She's also probably the best looking female NPC in the game, for what that's worth (not surprising, since she's basically the doll with a cooler outfit).
Orphan of Kos was excellent in gameplay terms. His moveset is quite diverse and the gameplay implementation of the phased battle is the opposite of Ludwig's in that the second phase is harder and more organically linked to the first phase. Both approaches are executed effectively, though, and both fights boast a similar level of overall difficulty (I'd give Ludwig a slight edge here though). That said, I found OoK's visual design to be underwhelming. I get that he's is supposed to be this ugly man-baby thing, but honestly, he just looks like a generic hollow from the first Dark Souls with a blue paintjob...
Laurence is a bit like OoK in that he combines great gameplay - and high difficulty - with disappointing visual design. Though at least in the case of Laurence there is some in-lore justification for the fact that he is just a Cleric Beast on fire, with that being that he was a apparently the first Cleric Beast.
The boss fights may be the highlight, but the level and encounter design were also on point, with my sole caveat being that The Hunter's Nightmare was the only level that came close to feeling sprawling, even if the Research Hall had plenty of nooks and crannies to explore. The Fishing Hamlet was kinda disappointing in terms of layout and complexity, but it definitely has the best atmosphere and art direction. Also, those Shark Giants are absolutely infuriating and the battle with the pair of them in the well easily rivals the hardest boss fights.
Overall, the DLC feels kinda small in terms of areas, even if you can indeed squeeze quite a bit of atmospheric and tense exploration out of it if you take your time. It probably helps that the DLC has some of the best - if not the best - itemization in the entire game, so there plenty of incentive to search every corner. The Pizza Cutter, in particular, is badass.
Honestly, the DLC enhances the game beyond the heights that it had already attained, which is frankly impressive. For years, I've viewed the Soulsborne games as roughly similar in quality, with some excelling in some things and some in others. However, playing Bloodborne again, with the DLC and after replaying the other Souls games quite a bit in the intervening years, allowed me to notice just how all-around tightly designed the game is, and the fact that it stands head and shoulders above the Dark Souls franchise. Not to mention it's probably one of the best games ever. In world-interconnectivity, atmosphere, and lore it rivals DS1. In level design (both peak quality and consistency), encounter design and itemization it's close to DS2. In terms of having a modern, smooth combat engine and tough boss fights it's comparable to DS3. The only aspect in which it is perhaps weaker than the others is probably the character system and build variety, since this was obviously the least "RPG-ish" of the bunch. But frankly, I don't hold that against it, especially since those RPG aspects could be pretty hit or miss in the others (except perhaps in DS2). Also, the art direction in this game is just gorgeous, one of the best in any game ever, and it's miles ahead of the Dark Souls games in this regard frankly. It's just an all-around great package with some fine highlights to boot.
Right now, I am positively hooked with the game again. The first thing I did after finishing my fresh playthrough+DLC was... to start another fresh playthrough with a more fine-tuned build. Which I have already beaten, and also the DLC. Perhaps I'll move on to NG+, but I also want to mess around with some other stuff as well. Perhaps the Chalices. Then again, I played the Chalices all the way up to Queen Yharnam back in 2015, and while it was a decent experience, I'm not too enthused with playing through them again. Haven't done co-op on my new playthroughs either, since it's kind of annoying to have to get PS Plus for that, but I do remember co-op in this game being fun. We'll see.