This build is mostly designed off of Azzapp's mobile Vel' Koz mid build from season 13. I feel as though Ludens is the best first item on Vel since it gives him the stats he needs most, mainly the small bit of CDR that is essential for weaving more than one combo in a fight. I don't know what the meta will settle into, however, I have noticed an abundance of hyper mobile characters and so I choose to go Cosmic second in order to get more of all of his essential stats and some insane survivability (since mage items for the most part no longer give HP, this will offset that and let you live much longer).

Late game, if you have a stacked Mejais and a Cosmic, even though you won't be dealing the insane damage that you used to have with Nightharvester, you will be throwing spells out non-stop and will be very hard to catch out due to rediculous move speeds.

Also, Malginance might be an acceptable substatute for Ludens (I haven't really tested since it doesn't sound viable). By doing this though, you waste a lot of that CD to your ult since it is already such a short cooldown with the runes and build, as we;; as really not needing Magic Pen on the ult since it should be dealing mostly true damage.