Firstly, those items are all ridiculously overpowered.
Secondly, they don't exactly fill a void. There are already items in the mana regen/CDR spot - Morello/Athenes. There are already items in the increase burst damage spot (Deathcap/Ludens).
These items are not similar to Duskblade, which fills a nieche other items don't.
Secondly, Riot doesn't want max CDR or a large amount of mana regen to be given without tradeoffs. Items like these do not have those tradeoffs (They increase trading damage by more than their counterparts would).
Thirdly, increasing the number of items doesn't necessarily increase the number of build options. With the two CDR/mana regen items we have now, only one is picked by almost all champions in virtually all situations. I rarely see Athene's. If you can't even make a choice between two items meaningful, how do you expect to cope with 5?
Fourthly, on your point about AP assassins, the itemisation situation is different between them and AD assassins.
AD items fall in to three main categories. Bruiser items, Crit multipliers (for those who focus on high hitting AAs), and the new assassin item line (Revamped Maw, Revaped Youmoo's, Duskblade). Bruiser items typically don't benefit assassins much (with the exclusion of BotRK). Nor do the major AA items. Beyond the core items AD assassins were effectively choosing which item would be least bad rather than which item would be good.
Compare this to AP items. AP items increase what your abilities do. You get extra effects - that brilliant Rylai's slow, Zhonya's active, an MR decreasing aura - and they increase your damage in slightly different ways - Luden's for burst, CDR for more sustained damge- but the items at their core all do the same thing - they increase how much damage your abilities do.
This means they work in effectively the same way for sustained damage dealers, burst mages, assassins and utility mages. An AP item made for an assassin could work just as well on a poke mage and vice cersa (see Luden's for an example). As such, there isn't a need for AP items to suit a particular class (except perhaps AP bruisers/tanks) as many of them already do.
Sorry for the long post. This is something I have been thinking about for a while.
