Snip
As far as I know, Osmose was a completely new materia(In Black Chocobo, it appears as "DO NOT USE"). So there might be some space.
Additionally, a lot of those abilities could be innates or tied to equipment.
Especially tying some of them to weapons would be interesting. For example, the listed debuff-effects could appear more often on Barret or Cait Sith, giving their attacks more utility(Those two are the tankiest characters out there, after all). Cid's current innate also feels very lackluster - I had it proc once so far and it is simply... boring. I am taking some inspiration here from another FF mod I enjoy(Brave new World), which had weapon quirks as a core part of it and the builds of characters. For example, spears had "auto-cover" on them and usually gave HP boni for lower strength, but with the option to double-wield them(Foregoing a shield for high offence). This made the setups very specific and unique.
One categorize weapons into similary unique categories(A current example would be Aerith's healing stuff and Vincent's weapons focusing on status effects). Bumping up the uniqueness of weapons even more drastically by giving them more complex differences is a space ripe for exploration.
Another possibility would be to add passive effects(unlocked via materia growth) onto a lot of the command materia(Like Morph, Steal) and refashioning the stats on them to fit the passive boni(Like the mentioned execute).
But~ We would need to know how much space there is actually left to add new materia first.
Another possibility would be, if UI and the like allow it, to add a new command similar to magic called "Skills", which would contain steal, morph, mug etc. etc. That way, you could slam steal and morph onto one materia and have both commands available at the same time. It would need to function similar to the "Magic" command we already have, but encompassing commands instead of spells.
Lastly, another possibility to differentiate characters more would be "personal" abilities.
Via levelling, story events, limit unlocking or whatever, a character could unlock more abilities, unique to himself. This is basically an extension of the "innate" system. Cid, for example, would learn stuff like "Execute", "Wild Swings" and "Focus". This system would add a whole new dimension of possibilities for unique characters. A strong, unique character base is a good thing. Currently, for example, my Barret and Cid are very similar: Physical Cover/Counter users with support magic and attack-enhancing abilities. Very similar base, difference being Cid is more damage focused and Barret is more defensive. If I decided to go for frontline Barret, though, Barret would probably just be a worse damage/defence hybrid than Cid.
A more complex innate system would "fix" that.
The main problem with a more complex innate system is, that it might inhibit build-variety... Shoe-Horning characters into certain roles. However, that is already the case for most characters anyway via their base stats(Don't try to build a mage-Barret, for example).