Sure, Wolven.  You can do this.  It'll be complex, but possible.

How I would do it is this:

Give the character 5 active abilities that they can see.  1) is Cast Spell which will unleash whatever spell you've created.  2-5) are Add Fire/Ice/Lit/Poison.

So, you cast 'Add Fire', then cast 'Add Poison', then cast 'Cast Spell' to unleash your fireball.

'Add Fire' (and the other elements) will just have a visual fx and add a stat.  You should hook the 4 elemental stats to your UI somehow to show the current elements you have mixing.

'Cast Spell' is going to be what's complicated.

It will essentially just be one EVENT_TRIGGER plus a STAT_WATCHER for each and every spell possibility you want for the class.  Plus, one EVENT_TRIGGER for every other possible combination that results in a fizzled spell (or not, you could just have it do nothing so the player doesn't waste their elemental points).

Now, here's where you need to decide something important.  Can the player invest skill points in individual spells like Fireball, Cone of Fire, etc?

If no, then all you need to do is have your 'Cast Spell' ability cast the appropriate spell.  If Yes, then...

Two things.  First, you need skills players can invest points into to increase the power of their, say, 'Fireball'.  These will actually be passive abilities and not the actual skill that gets cast when they hit 'Cast Spell'.  The passive skills will just increment a stat, 'fireball_level'.

You now need to have 'Cast Spell' execute an intermediary spell, say, 'Fireball Level Check'.

'Fireball Level Check' has 15 EVENT_TRIGGERs each with its own STAT_WATCHER which checks the player's 'fireball_level' stat so that it executes only the appropriate level of 'Fireball'.



So, a lot of work, but easy enough.

Of course, it would be at this point that I strongly suggest ditching the skill investment model that vanilla TL2 gives us.  I think it sucks.  I think we should move to a single-level active skill model a la WoW or Path of Exile.