That would give it a 2400 gold value WITH THAT PART OF THE BUFF ALONE. And replace ghost as a summoner spell entirely for champs that have to run it to be effective.
This would grossly overpower several champs who must either take ghost as one of their SSs (granting them a huge power spike by enabling flash/exhaust/ignite allowing them to also ignore many other suboptimal utility items) or build phantom dancer for the ghosted effect.
This would just be way to insanely broken (unit collision abuse is a very important mechanic in game and what makes champs like alistar, fizz, hecarim, (and Maokai wall/turret pin abuse) etc SO strong.
You even admired it in your own post, unit collision keeps some VERY powerful champions balanced/from snowballing uncontrollably.
Don't forget a ghosted unit can also 'hide' in a larger modeled unit PREVENTING you from targeting them entirely (or a larger modeled unit that's ghosted can 'protect' a smaller unit).
Considering the majority of champions that use this Key Stone Mastery ARE ALREADY THOSE CHAMPS this would be utterly busted.
No it will never happen (again a gold value of AT LEAST 2400 gold on a single extra effect on the keystone (not counting the bonus move speed and slow resistance), 20% life steal is only 1800(actually 1200) for the WHOLE mastery, CoC has 1820 or so, thunder lords is too variable to calculate, wind speakers has about 1600-1800 etc.).
TL;DR: it would give the mastery the value of an entire summoner spell on a vastly shorter CD or an estimated total gold value of 5400-8900 gold (depending on if vs slows or not) easily EXCEEDIBG the value of a summoner spell AND an entire 2400-3800 gold worth of items (and that's stretching it it's gold value is actually a lot higher in optimal situations on said champs if this change were to happen).
If it's ONLY function was no unit collision and 100% slow resistance it would be balanced but 70% reduction + a move speed bonus equivalent to ghost AND actually ghosting you (a far superior ghost spell on shorter CD).
I dunno campaigning for the Ghost summoner spell to slow 70-100% complete slow immunity and 24-35% movespeed over time (champion level) would be a more realistic request to pursue (if not equally unlikely as ghost is already so strong on such champs because of the lack of unit collision thing).
Anecdote: Did you know Annie can ghost and hide inside Tibbers while ghosted PREVENTING HER FROM TAKING NON AOE DAMAGE? It's like having tahm kench eat you except you can still attack!
DID YOU KNOW ALISTAR DOING THIS TRICK FOR MANY CHAMPIONS is what inspired tahm kench's ability? DID YOU KNOW you can't attack while tahm kench has you eaten BECAUSE IT WOULD BE TOO BUSTED? Do you realize you can still exploit this and it's been exploited since Ghost was put in game? Do you start to understand why this can never be a mastery with such easy availability?
(There are also programming issues and bugs with granting this on a mastery. There is also the unit collision pinning that happens when a ghosted champ ceases being ghosted (they get stuck or can get other units without a blink PERMANENLY STUCK except for death/recall) its very game breaking/ruining/unhealthy when it happens... also some issues with clipping through the map and being removed from game could become very rampant. It would just be this huge can of worms :/