Agreed with you all: The factions in Elemental need far more personality, especially factions of the same ideology. What is the difference between Altar and Capitar, or between Resoln and Umber? The aesthetic spark is missing in the different factions. To this day, I can still remember most all the different factions of Galactic Civilizations, despite not having played it for years. The warmongering Drengin and the calculating Yor, the greedy Korx and the mysterious Thalans; they all had a unique flavor and distinguished themselves in my mind, despite the fact they all had the same tech tree in GCII (Vanilla). In contrast, I have a hard time even remembering what factions are in the game, let alone what their own unique styles are. Increased personality is certainly in order. Unique Architecture and weaponry would certainly help. If the buildings of Gilden sported hard edges and mighty pillars, while the Tarthians used earthen foundations and roughshod designs, they would feel more unique. If the Kraxis attacked with dark plate armor and serated blades, while the Yithril used intimidating pikes and chain link armor, the two would feel more different.
An interesting dilemma is the issue of different races. Pulling out the orcs and the elves, staples of fantasy worlds, would be an easy way to differentiate and add spice to the different factions (if not too easy, but I digress). Yet as Kellhound_AU pointed out, only two basic sentient race types exist in the lore of Elemental: humans and the human-like fallen. There's no room in the lore for all of the different species we might have in mind. And besides, the goal is to improve existing factions, not replace them. So what can we do? Aside from reimagining some of the races, I would propose that we could have our cake and eat it too by giving each kingdom and empire their own unique sub-factions. Perhaps the Tarthians, living so long in the forest, have a natural connection with the woodland nymphs, and the tunneling Gilden have alone uncovered the long-slumbering Drath from within the mountains. Maybe the Magnar, consumed with desire for knowledge of magic, have learned to bend the wills of Elementals, and Resoln has learned to tame the spider as Capitar and Altar have the horse. These subfactions would begin small and weak, but as the kingdom/empire grows, so too would the strength of these subfactions, and they would make a greater appearance in your army (Mounted spiders, Drath soldiers mixed with Gilden troops in the same unit, etc.). Giving each faction their own unique subfaction would help make their backstory an active part of the game and provide each faction with more unique flavor.
