I like Darvin's idea for Platemail giving slow status. Heavy armor is heavy, after all. (I'm too lazy to check - does slow status prevent you from getting bonus XP from a slowed enemy? If the plate + RBS combo were too powerful, disabling that source of XP could be a fair tradeoff.)
It doesn't prevent bonus XP. Big thing with slowed status is that it stops you hitting any status effect enemies (poison, MB, Curse, Weaken and Corrosion) and it means you *have* (if you have a slow/first strike thing) to be able to tank an extra hit from any enemy you want to take down, which kind of defeats the point of using platemail in the first place. Unlike RBS of course, you couldn't convert it mid run without paying replacement costs.
Any thoughts about the Crystal Ball? I've been trying it out quite frequently on my new profile, but find that it rarely pays off. I might use it once relatively early on to get a high level kill, then save it for the boss and either use it once (in a hard dungeon) or find that I don't need it at all (everywhere else). Granted, I haven't gotten to the vicious dungeons yet, and of course I nearly always go purist, so my perspective is limited.
Still, the gold cost and the fact that it takes so long to recharge make it hard to pull off even two uses, and when you're only using it twice it's just not worth it. 25 gold for 2 full MP restores, but you have to leave it sitting in your inventory for basically the entire dungeon, versus 15 gold for an easy-to-use and easy-to-store Schadenfreude or 20 gold for a Keg of Mana isn't a very good deal. Lower the use cost to 2 or 3 gold and it'll be a lot more reasonable, I think. (If that makes it too strong, you could always put a cap on the number of charges, which would also encourage more frequent use...)
Crystal Ball remains great in my books. Like all the gold items you get more use out of it in Vicious, and recycling some of the mana from your spellcasting is really valuable in scenarios when your resources are pushed more. (as all these spikey types of items, it's stackable with all the other ones - a typical situation with a charged Crystal Ball w. 10 gold, 5 MP fireballs and 25 base mana is on its own 30 mana/6 fireballs. However, every other source of MP restoration (bloodtopowa, Schadenfreude, MP, Clearance and Refreshment) adds a further +20% mana with the crystal ball (so with schadenfreude, 3 mana potions and 2 refreshments, you'd get around 45 from that crystal ball).