Honestly, if they do ever add in another summon, they need to do it as a base Arcanist summon that then has different versions for SMN and SCH in the same way they did with summon and summon II. The way I'd personally do it is give us a "diamond" carbuncle for the base class, (this could be that white carbuncle that was always shown in promotional art for the class), that is sort of like a middle ground between Emerald and Topaz, with 1.5 times the player's health, (rather than Topaz's 2 times), and with DoTs and debuffs. The basic attack of it could have a slight enimty increasing effect, like Topaz, but it wouldn't have the other abilities of the tanking pet, (AoE enemity gain, defensive buff, and stun). This would of course carry over to the SMN version, making it viable for both solo play and group play, (because at that point if a tank can't keep the enemity from a tank pet, they're not doing their job), while still making it different from the current ones. More of a middle ground for when you need damage at the same time you need your pet to survive, but also with a debuff or two to make it still unique in it's role.....

The SCH version would be a pet that focuses purely on buffs of some sort. It would have the basic healing obviously, but would have some sort of esuna/leeches spell as it's second ability, and it's long cooldowns could be a defense buff of some sort, and a damage buff.....

Just my thoughts on the matter, and in truth a bit irrelevant because it's unlikely we'll be seeing any new summons added to the game. My main point about making it a base skill though is that it would be a bit unfair for SMN to have even more summons while SCH is still left with only two, and it would also be pointless to simply give them both a new individual summon skill when they could make it a base skill as they have done before. Plus, I want that white carbuncle.....