This.
Me personally, I'm partly with:
I personally like the "one and done" summons rather than the "stay on the battlefield" ones, since you get the classical Final Fantasy feel of summoning this big attack and you get your personal abilities changed for a short time to kind of go with that and mix things up with different elements/themes to prevent it feeling like you're just casting the same spell over and over. I personally like that a lot.
I do think it would be cool to have some differences in the summons...but part of this is FFXIV's rigid combat system. For example, it would be cool if we had, say, Shiva and Ifrit (DoTs) for single target attacks, Leviathan and Garuda (DoTs) for AOE attacks, Titan to give the party a shield (like FFTactics Golem), Ramuh for party buffs, and maybe something like Moogle/Faerie (both have been used in the FF series for this) for party healing. Then this builds some gauge over time that lets you use "the big summons" of Bahamut, Phoenix, Alexander, and so on, either in a rotation or with different effects as well.
...problem is, FFXIV's combat system is too rigid for this. While FFTactics Summoners could summon elemental attacks, bigger elemental attackers, one-hit KO chance (Odin), big nonelemental attackers (Bhamaut), healing (Moogle/Slyph and I think Unicorn?), and defensive (Golem) utility summons...well, that wouldn't work as well with FFXIV's super rigid combat system today, where SMN isn't even allowed to cast Radient Aegis on others (except in PvP...). I mean, sure WAR can be a Tank and Healer and marginal DPSer, but...that's not supposed to happen, and RDM is hardly replacing the party healer anytime soon other than picking people up off the ground.
So my second best choice would be to have several "lesser" Primals (like Titan, Ifrit, Garuda, etc) and you can pick to use whichever. My ideal would be to allow people to use the same one several times, even. Like say there's a high movement fight. Maybe I want to go Titan, Titan, Titan. Maybe Ifrit does more damage, so if there's a fight (or phase of) where I can stand still, he'd be the optimal one to use. Maybe Garuda places a 40 or 60 sec DoT on the enemy such that I want use her every other/third/whatever summon. And after using a set number of these (say 3), you summon the big ones (Bahamut, Alexander, etc) to do some bursty damage. The fight would start with the big one ready to go (just like now) for the opener, of course.
I'd like the flexibility that would offer, and also being able to use a fourth "lesser" Summon in some cases, like when the party needs to hold burst for 10-20 sec, something SMN can't deal with now without getting out of sync with the rest of the party.
I think that's what I'd like at the moment.
In lieu of THAT, to have the other three ARR Primals (Levi, Ramuh, Shiva) after Phoenix. Ideally with a bit different playstyle, but even if it was just Titan/Garuda/Ifrit reskinned, I think it would still be a nice aesthetic change of pace.