I don't think you are missing much with any of those games tbh. Unpopular view but so be it. Kotor2 is ok, or it was in 2000 or whenever it was made but playing it now wouldn't feel worthwhile imo, I tried. Also in terms of tactical combat which is the only thing I care about in any RPG, it is very simple and RPG-lite. Still, it is the best thing in that list. I don't like Fallouts, Arcanum is a shitshow, Elder Scrolls/Witchers/Dark Souls are dumb action games that don't even have good action, Final Fantasy is for little girls and story fags, and vampires are for goth kids, which I could put up with if the gameplay was good but it just isn't. I think 3d RPGs can be great, the problem is it usually goes hand in hand with making it dumbed down and more of a glorified FPS. There are exceptions that are worth trying though, System Shock and sequel, Deus Ex, etc. Also Final Fantasy Tactics is great.
And before the omg wot DO u liek?! question, I really love KOTC, Krondor, JA2, BG2, IWD2, TOEE, EQ, Divine Divinity, and some others I forgot. And I had some ok fun with MOTB, DA Origins, DoS, Wiz8, the kung fu Bioware game, and some others I forgot. I love the System Shock games too but I don't think of them as RPGs, just more interesting FPSs.
p.s. But if you like Plansecape you probably have a different point of view to me.