Mezek is certainly an interesting card to build around.
Only drawback is his CMC imo. At cost 6 he's not a quick drop, certainly not for blue/red.
If you can get him in play around T3 then it becomes interesting.
Grim Monolith maybe ...
In your deck, just one copy is not enough.
More often than not he's not going to show as you have no search cards or tutors (
Imperial Recruiter )
Also consider instants and sorceries with Aftermath and Flashback.
Does this deck have a reliable T3/T4 kill ?
If not, it's not going to hold up in Legacy.
As a rule of thumb for Legacy, whatever you build has to be able to reliably kill or control the game by T3/T4 in Goldfish mode.