The new AP setup appears to be...
Masteries - this is the only one I'm having doubts with. Currently I've been running 18-12-0 but 12-18-0 seems to be popular too. I know that on DoT mages, heavy poke, or mages with lots of single target damage (or Brand who literally has a little of all 3 of these and is the most broken thing this patch), 18-12-0 is strictly superior.
Runes - This isn't the hugest of changes. Mpen marks or dual pen, depending on how hard you auto-harass with your champion. I'd advocate dual pen purely for how snowbally games are, to the point that little bit of mpen you're losing out on late-game isn't relevant. However, games still last long enough that I strongly recommend HP/lvl over flat HP. Flat AP/MS Quintessences still reign surpreme. The biggest change has to be glyphs. AP/lvl or flat MR are going to be your best choices, with options for flat CDR or in a very few choices (such as TF) CDR/lvl.
Items - So why a shift from CDR/lvl? Because Ionian Boots. 800 gold. 10% basic ability AND summoner CDR. At the start of the season I thought it'd be Sorc Shoes still to deal with the absurd MR values on SV and BV. But here's the thing; nobody is rushing them except tanks. You know, the guys you (as the AP mid, maybe top) should NOT be focusing. Mercury Treads are weak right now, so even bruisers won't want those over Tabis to deal with the ridiculous changes to marksmen. What about their rush items? They're busy playing with new toys such as new Trinity Force, Guinsoo's, Ravenous Hydra (you can be damn sure Renekton's going to get this now that he can easily swap it off for Titanic in the late-game and still be relevant), etc. Hell, I normally don't see bruisers even get MR/armor outside of Tabis until 3rd item now because of stuff like Sterak's. Most of their tankiness is just health. This is also why Void remains an optimal buy AFTER Rabadon's.
Ranting and going off on a tangent aside, the new build should look like your standard build before, but substituting Sorcerer's Shoes for Ionian Boots.
What does this mean for your AP champs? TF still gets CDR/lvl because this was his build before, and it only gave him 25% CDR. Now it gives 35% because of Lich Bane's buffs. I feel this is also an indirect buff to champions like Karthus and Rumble because it effectively allows them to itemize CDR without truly sacrificing a substantial amount of damage. Rumble's favorite item (Liandry's) is that much better since there's very little substance to most bruisers' tankiness in the early game (which, again, comes mostly from health). He also abuses DFT pretty well.
Blatantly overpowered champions right now are 
and 
Champions you should definitely look for and/or are underrated right now, due to their synergies with the new items/setups would be...
- Lots of DoT and a fair amount of single-target. I would say this is one of the few champs I'm iffy about on CDR since she doesn't get as much out of it and she already gets plenty from her passive.
- His E is a strong DoT and his Q is coded as a single target spell when he strikes a single target with it. Definitely should look into him.
- I cannot stress how abusive these changes are for him.
- Nah not really this is just to see who is actually paying attention when they comment.
- Swain Swain Swain Swain. I'm starting to notice a pattern of how the DoT mages are all particularly immobile.
- Lich Bane got a buff. +10% CDR. I mean, it's not like Twisted Fate will ever find that useful. Nor the Ionian Boot buff. /sarcasm
TF has great synergy with two of the better items right now. The downside is he doesn't do as well proccing Deathfire Touch as he really only has one ability that procs it, iirc. I don't believe Pick A Card works with it, because it's an AA modifier. Probably better to run 12-18-0 with him.
- As a champion with multiple reapplications of damage, Viktor got a buff via Deathfire Touch. As a champion who abuses Ionian Boots and Lich Bane as well, Viktor got a buff.
Overall Viktor is really amazing right now with these two changes. He damn near was about to fall off the radar before this patch hit and he got a second wind.
And of course, you've got the ever popular solo queue champs
Hope this helps.
