For EA Agartha, I think it would be cool if it were something like the Balrog from LOTR.
That would be an incredibly cool summons to see, if not for them, than for Abysia perhaps.
Personally, I think there should be some powerful endgame summons available for every path, if not for every nation. Currently only a few nations are blessed with an abundance of potentially useful summons, leading to the S/D/B paradigm that we see today. It's true that there are -some- decent summons in other paths, but many are considered inferior, and too many are unique, which means they'll never be competitive against even 5-6 Tarts, let alone a dozen or more (even with Shattered Soul, sheer numbers tip the scales).
The only problem that I see, is that if a mod is developed, it's likely that it won't be widely adopted, even if it makes great strides in making more playstyles viable. Take CBM for example, I have tried it, and most of the changes are pretty awesome, as far as widening strategic viability. Yet, most games do not use it, so getting used to those options has no value in MP. In fact it could be argued that while content mods are great for making SP more fun, that they are detrimental to anyone who enjoys MP, as they teach you a different game.
The flip side of that, is that there is the remote chance, that if the summons were thought out well enough, and received widespread acceptance, IW might, MIGHT be persuaded to include them in the game, and that would be priceless indeed.
To be perfectly honest, I think IW could easily fill in the gaps by taking the pretenders as they exist, adding a couple magic paths (to most of them, not all), changing the descrip, and then making them available in the appropriate paths. Removing the conceptual and graphical phases of the creation, should relatively simplify the design of the spell/creature, and there is nothing implicitly unique about any of them really, the pretender is just a very ambitious individual.
