What Does Group Slug Need
Its partially because there are better things to be doing in mono-blue (if you want to focus on fast consistent wins, that is), but I do think Cosima can offer a lot, particularly if built around
As long as Awakened Skyclave is on the battlefield, it's a land in addition to its other types
And all of the abilities are appropriately cost When costs one more than Remove Soul, What is one more than Shatter, etc
Meloku is a great late game rip off the top and will win most games where he gets to live for a couple of turns however he is reasonably easy to deal with and offers little value when played when you first hit 5 mana and is then immediately dealt with
All that said, if your local group gets agreement on using them (or any other House Rule, quite honestly), we support you in doing so