I propose, with some wiggle room:
“Your next 6-8 spells (optional: within 8 seconds) will shoot two additional orbs dealing 30-35% damage each”.
Basically exactly the way it works now, but make the first 6 spells have 35%x2 damage boost then end the triplicity effect.
You could also extend the 15 spells required to cast triplicity it to something higher. 20-25 maybe?
I cast triplicity extremely quickly imo. I maybe could shave some time. It’s almost 1 second on the dot, and I feel at best(honing & improving the cast) I could do 0.8 maybe. Comfortably I cast it slightly over 1 second. I imagine this is similar to other mages. In this time, I can cast more than 2 fireballs. Thus, triplicity must be worth more than 2 fireballs at minimum for me to be interested in casting it. 35%x6x2 = 4.2 fireballs, at the cost of about 2.25 fireballs for your typical end-game mage. Netting us a profit of 1.95 fireballs per triplicity. At 5 spells, instead of 6, we would get 1.25 fireball profit. Every 15-25 spells. I am assuming here, despite having <2 seconds on 3 second tiles after triplicity, that mages would adapt to have their full tiles.
Momentarily, let’s assume those 15-25 spells are just fireballs. After 25 fireball casts and immediately casting triplicity with the 6-spell version, we’d get about 6% dps increase(32.95/31). At 5 spells(31.25/30) - yes decimals and quotients! – we get about a 4% increase. Note that we literally do not have a usable level 30 talent right now lol. A 4-6% (for a 100% perfect rotation, using triplicity as soon as it is ready) increase is good, no doubt…but level 30 talent. I’m not gonna bust out the math for a regular rotation, but just to provide an idea. Also ignoring casting triplicity at spell 24 
The above also assumes 2.25 fireballs/second and a 1 second cast time of triplicity… increased triplicity cast times would lower the % (as for ex in 1.5 seconds you could have casted 2.875 fireballs @ 2.25/second and so-on). Boosting 8 spells for example, at 30%x2 each would be 4.8 extra fireballs instead of 4.2 from 6*.35*2 and may be more appropriate when considering rotations and varying triplicity casting times. However more spells buffed means more casting, meaning the further away we get from keeping up tilesets, potentially resulting in DPS loss & making triplicity less attractive. A 2.25/s mage can cast 9 fireballs in 4 seconds. The other option would be to keep 5-6 spells buffed, not 8, and using a lower number of spells required to cast triplicity(15/20/25) to increase overall DPS but more spread out over time.
As for why triplicity is currently a total wreck regarding tiles… long story short, it does not play well with tilesets at all, lasts an entire 8 seconds, causing further DPS loss.
Pretty please? Can we have a level 30 talent? Some different shapes to draw in the sky?
Reference: Runemage Triplicity Talent Rework