I honestly don't think adding another 3 Arcanum summons will change anything at all, unless they also change how the ebb and flow of the job works. We have already seen this with the additon of Bahamut Zero, functionally, nothing changes just visuals. There is potenc differnce between bahamut, and Bahamut Zero but unltimately it changes nothing.
I am all for variety, heck id even go as far to say add the rest of the primals onwards, however, i still think unless they functionally change how the job works, adding new summons will literally change nothing but visuals.
The issue is and has never been a summon variety issue, but rather a mechanical one. The base machanics of the job needs updating to accomodate summons in a way so that they are functionally different and not just thunder Garuda, Ice titan or Water Ifrit.
If they cant do that then all they are giving is reskins which is effectively the same change they gave us in Dawntrail. IMO if this is the case they may as well not add anything at all.
So whats the solution?
IMO its redesigning the job so every action is summoning or relating to powering up summons. Ruin for example could call carbucles to attack and then de-summon. The demi could rotate, but its power is derrived by the number of arcanum summons that have been used. Each arcanum summon could also add for a filler.
I think there would need to be a mechanic that allows us to build a resource to spend on summons. Ina system like this, it would be really dynamic and choices would matter. Id go as far to add a true Demi Hydelyn and Demi Zodiark but have them available based on the use of 6 arcanum summons.
I would also have it so the carbuncle you summon attacks and you have off spec special attacks you can command it to do. Id even go as far as allowing it to be the form of multiple Summons and have its ability changed to match.
IF we stuck with the Garduda, titan and ifrit, then perhaps have a grand summon option where that arcanum gets powered up into the SHB raid versions. (even if we didnt stick to the three id go with this) Tier 1 - current > tier 2 shb > tier 3 > Warring triad. But functionally it would allow for planning, build and spending of resource.
Im probably not explaining this well, but bottom line is i want a summoner to feel like it summons, and that those summons are a meaningful choice. With the flexibility to decide not to summon, or to change what summon is there or to empower the summon in some way.