I'm designing a new setting, a sort of post-apocalyptic futuristic world, somewhat similar to the old cartoon Thundarr the Barbarian. For this setting, I'm creating mostly rebuilt or new classes. Instead of actual magic, there are categories of esper, essentially people with psychic gifts, although I'll be adapting spells to replicate this. They'll be divided into three specialized types -each limited to a specific theme. They'll also be using the bard spell progression (-1 known), so limited to spells of level 6 at best.
The themes are Energy/damaging matter, mental control/minions, and health/bodily improvement.
Do you think the game world will suffer for not having access to 7 - 9 spells? Would you consider playing a "caster" to be worth it if they were narrowly focused? Do you think this will reduce the inherent risk of caster dominance as levels climb?
(Note, the setting will only have one category of creature that is outright immune to enchantment effects, robots -constructs of course- so being focused on mind affecting powers won't be as problematic as in the base game)
(Edit note: I'm sure the DSP psionics is good, but I'm not interested in it for this)
Many folks do iterations of eX and seem to enjoy it; I'm not one of them.
However, a lot of what I've read and personal experience with casters as a GM, the game will not suffer for the lost of spell levels 7-9. Even with 6th level spells, casters are quite powerful.
Are you yanking these spells from creatures, as well?
Let me begin by saying that the supposed martial-caster disparity has rarely shown itself in my own games. If your choice to eliminate full casting is due to a belief that it is overpowered, then I disagree with your decision. If, however, you've made this choice is for thematic reasons that have to do with the setting you are building, then I approve wholeheartedly.
With that said, my experience with 2/3 caster classes has generally been positive; they can bring a unique feel to spellcasters, who can feel very similar to each other despite their plethora of options. Without a full casting progression, there is a fair amount of room for interesting abilities that can give them a unique playstyle. For your specific choices, I might enjoy a blaster mage that actually works, and the enchanter style character has always been a favorite of mine. I've yet to see a self-buffing style mage that I thought was interesting, but I'm sure it can be done.
From a mechanical standpoint, I doubt that the game will suffer at all from a loss of the highest tiers of spells. In most games, those spells are encountered very rarely as it is (I can't, for example, recall the last time I actually looked up an 8th level spell for use in a game). If you need a specific spell or effect as the DM, it is always easy to include it as a special or unique ability. In that case it may even be even more memorable for your group, knowing that they have encountered something truly unusual. If you believe that the "inherent risk of caster dominance" exists at high levels, then this is one of many ways to solve that problem, certainly. It is probably one of the more thorough and elegant solutions, assuming your classes are well-built and interesting.
From what I've seen of your posts in the past, I won't worry too much about that last assumption.