I like the Idea posted by the OP.

Some of the Skills which SMN loses, such as the DoT skills, fester, bane as well as the Ruin Spells should be given to SCH, maybe not with the same potencies, but it would clearly make the job more interesting, as it is now.

As for Egis, i think a good way to design them would be to give them a "enrage bar" that fills up during combat, as long as the egi is out, once it "enrages", it can take its demi-form for a limited time and gets a new set of skills. In order to properly balance this, it would require Egis to be killable again (as well as updating the way they are controlled). The Demi Summon also gets additional stats such as increased HP and stats befitting its Role.