Disclaimer: Yes, I start off talking PVE, but I get into PVP eventually. I've thought this through pretty thoroughly, and am not suggesting anything that is unbalanced from a PVP standpoint.
PART 1: Introduction to Summoner Destiny, and the truth about the synergy of summoner classes:
Let me preface this by saying I've spent extensive time playing around with a summoner destiny, just for fun, and have found incredible PVE success! (PVP is another story and I don't want to talk about my crushing defeats.) While many people find the classes are downright contradictory due to Bonder's emphasis on Bonded Youkai, meaning you're restricted to just 3 as a dedicated summoner... the actual truth of the matter is that Bonder does desperately want some GS skills, and vice versa. The problem is that, combining these two classes without destiny is basically better than with destiny, but I'll get to that soon. For now, here's the skills that each Summoner Promo wants from each other:
From GS, Bonder Wants: Spirit Regeneration, Reihou, Evoke Mastery, Encourage, Impact Dismiss, and Absorb Ether.
From Bonder, GS Wants: Healing Discharge, Fast Offense
Why do I say these? Bonder has fewer youkai to use, and only has Healing Discharge/Energy Transfer for sustain. If a youkai goes down in a long BDP, all they have is Bloody Hearts (Generally luck based drops) or leaving to use an inn. On the flipside, a GS is much more sustainable over time since it can let a youkai rest for a bit, and can even revive them! However, it lacks similar support for already- summoned Youkai.
Now that we've established that these classes do have skills that compliment one another, let's move on:
PART 2: The reason Destiny isn't even remotely worth it, and you should just use both normally:
Why do I say that? Let's take a look at the amount of skills each class has, and the synergy between them:
Bonder: Only 52 SP worth of total skills, several of which can only be used on the same build with hybrid weapons, and the vast majority of which being linked to bonded youkai. You don't even need destiny to basically get everything you'd likely want as a Bonder, and Destiny leaves you with SP left over! If you want to use more than 3 Youkai, most of your skills are 100% useless; you can only use Ritual Sword, Aligamato, Fast Offense, and Healing Discharge. Along with their passives, this is only 23-26 SP total. OUCH!
Grand Summoner: 71 total available SP worth of skills. However, remove 5 for Summon Storm since it's pointless when you can summon all Youkai for 1/3 of the FP cost in the same amount of momentum when limiting yourself to 3. Remove 5 for Spirit Well since it's a joke of a passive. Consider if it's worth locking yourself out of one of only 3 youkai for 4 turns with Spirit Chains, and if you deem it not to be, then also skip Youkai Tamer. Aegis Shift is useless in PVE since enemies will literally ignore Youkai just to come for you, and your Impact Call may be detrimental, so there's another 6 wasted. Also, remove 1 for Auto Summon since you'll have enough skill slots for evereything, most likely. Also, I've got 5 allocated to New Heights since I'm not sure if Bonder's passives alone will stave off the hextupled costs, since both classes are in use. So basically, you potentially might not even need over 40 SP to get everything you want... again, what benefit is there to Destiny, really?
I'm sure you see the problem. And that's just getting started. At least without Destiny, you have freedom to try something else in...
Part 3: Summoner and PVP: The final 50% of the nails in a Summoner Destiny's coffin.
Hoo boy. here we go. Youkai can be a nice little extra to have in PVP. Either to serve as a distraction or a bit of free extra damage, and a player with high level youkai can put the evoke skills to use with their better stats. There's a lot of potential synergy, like Seiryu and Lilu to increase damage dealt while simultaneously reducing damage taken. And I'm sure I don't even need to mention Ingrain + Adversity Tunic in some scenarios; I've personally used this on a Bonder/Priest to slaughter a Lich GS's entire 12 youkai army, Pre-GR. (Yay for Mercalan Mist + Holy Spark when surrounded by that many targets) Point being, Summoners aren't actually so bad in PVP.
Here's the problem, though: The reason summoners work in PVP is generally because the character brings more to the table than just the summoning. Unless using some kind of hilarious gimmick build, like a River Sword + Parraeta Spam (no, this does not increase the drain, just the raw damage output) the summoner aspect of the class is generally more for utility alongside another class. Even if summoning is the main focus, it's generally more beneficial to them to carry another class for utility (Such as perhaps Curate to buff the summons, or more recently, Engineer to kit the Youkai out with Jetpacks and Electroshields - I've done this and it's hilarious fun) instead of the other summoner promotion.
Part 4: "Seems pretty shit. What do you possibly think could help besides adding new promos that DO synergize better?"
New skills for each promotion: This will add more synergy while also giving you more places to spend SP to make Destiny worthwhile.
- Give Bonder something akin to Spirit Regeneration and Evoke Mastery. For a Destiny that's all about the Youkai, being able to stack like effects such as Spirit Regeneration from both promos just makes perfect sense, as a completely dedicated Summoner. (We already see things like this in other classes, like Kensei and Ghost stacking their crit bonuses, etc.) There's another 10 SP to spend right there, but dedicated PVP bonders won't care to change their existing builds since these are more PVE-oriented sustain effects anyways.
- A GS skill that allows Youkai Bonding in larger numbers: Up to 8 total, instead of 3. +1 per rank, 5 ranks. (So Riagri can attack every direction, like Famiuga.) It's 100% useless outside of being paired with Bonder, but it allows a Summoner Destiny much more freedom than before. It also lets you field a non-gimped Carbuncle, which is nice. Keep in mind, this also locks you out of those great "Only one bonded youkai" bonuses! Still, a small price to pay for removing the massive barrier that keeps most people away from Summoner Destiny at all.
Honestly? I think this is actually all that needs to be done for now.
Pretty decent ideas, but I think for Destiny to be worth we'll have to wait what Dev has hidden in his sleeves for the third Summoner Promo, it will solve this one in particular imo.