Lack of high AP + CDR items

So I tested out the new runes on the PBE (primarily on my Xerath), and the maximum of 10% CDR (that works towards your cap) is really frustrating. Currently on live, I typically build and have 20% scaling CDR in my runes. This allows me to be the glass cannon I want to be.

On the pbe however, I can only get up to 10% CDR in my runes, leaving me short 10% CDR. Now I'll have to sacrifice one of my heavy damage AP items for and/or or my for . My issue with this is it basically removes the option to play as a full-on glass cannon, a playstyle I greatly enjoy.

Building defensive should be a choice. Even if it was always optimal to build some defense, I should be allowed to choose not to. To solve this, could you please add a heavy AP item with some CDR on it when you update mage items?

Edit: As I've said several times already, I'm not asking for everything. I'm already giving up defense here. Why should I have to give up CDR too? If you want to balance it out, I'd happily throw out the HP on Liandry's and tack on the CDR there. That would be a cost for the CDR.

I mean why should a champion like Xerath who gains next to nothing from defensive items be forced to buy defense to have an optimal offense? That makes absolutely no sense.