Rylais feels like a wasted slot
tldr: Rylais is not gold efficient, has no significant stats and there are better options in most cases. Rework it into something useful.
I agree that the slow nerf was needed (6.24), but the stats (75 ap and 300 hp) feels underwhelming and there are no actual reasons to build this item..
Mages usually get this as a semi defensive item to counter champions that engage onto them. The problem is that most mages already have some type of slow in their kit (or runes). There is no reason to waste a complete slot on this for a measly 20% slow and 300 hp?? Liandrys is a far better option with similar stats, as it gives defense and offense (magic pen and %hp burn) at the same time.
This item used to give a huge spike with 100 ap and 400 hp and massive (op) slow, but was nerfed and reworked to feel like an early game utlility item. Buying it early is not a good idea though, it has no mana or cdr and the defensive portion is not that significant. Morello and ROA are far better and more gold efficient options at the same price class.
If this item is supposed to be in the utlity class, along with zhonyas, Banshees veil and hextech items... then i suggest you add some cdr, mana or make it into a truly defensive item.
AD champions have a complete set of items and runes to shut down mages in any situation: merc boots, hexdrinker/maw, nullifying orb and mercurial schimitar, even wits end on certain champions. But mages can only buy zhonyas and add 5 armor from runes, they really need more armor items.
As and early game utlity item: 80 ap 300 hp 10% cdr Price: 2900+ g
or: 80 ap 300 hp 300 mana Price: 2900+ g
OR as a truly defensive option for mages against adcs, assassin and melee carries (yes this is biased): 60 ap 300 hp 40 armor Passive with AD shield or Attack Speed slow (Fits with the rylais crystal/slow theme) 3000 g
Edit: Tbh i think the slow is fine as it is now, its purpose is to improve sticking power and kiting power on lowrange mages.
If the slow on rylais was to be stronger, say 40 or 50%, it would have to trigger like ludens echo, where you could charge it up with some mechanic (moving, casting spells etc).
Or give it an internal cooldown, so the first spell hit will slow every 4 seconds, so the target also has some counterplay. This way you could have the item be healthy with a strong slow, but prevent abuse and permaslow on some champs._
: Speak for yourself.
now and
when it was good are terrible to play against because if you can't kill the other guy you won't be able to escape them either.
, and Rylai's being core on Morde, because ... you know... no CC or mobility.
+ 

That's about it

(AP Kog)
all benefit from buying it. So it isn't really a wasted slot. It's just one with a specific niche.