I'm doing a bit of investigation for my own article and here's what I found.
Every single life summoner has been defensive in nature aside from General Sloan, who just came out. Maybe that means they're ready to open life up to more offensive capability or maybe sloan was a one off and we should stick to defensive ideas.
Personally, I'm leaning towards the latter and I think he could be more like a Cleric or a Priest than a Mage / Wizard which would have me thinking about magical ways to be defensive.
So I like your idea of Phase and + speed, defensive utility that he enhances his monsters with through magical spells or artifacts. I'm also thinking Forcefield could be in play as he shields the whole team with magic and it hasn't been done before.
Originally I thought we could see something like +1 melee attack, +2 HP, and a defensive ability like divine shield because when you look at the melee life units in Chaos Legion, they have pretty poor damage with low hp (Sunkai Harvester, Chaos Knight, etc) and that it felt like they were designed a bit on the low side so they could be brought up by the legendary summoner but the art has put a damper on that theory.
Always fun to speculate!
Really thorough, I love it! Now that you mention it, his white cape does give off "cleric" vibes...