hmmm, PB uses of free [associated element] transmuted spell feels a little toothless to me (if i understood correctly what you were suggesting), maybe split the difference and say you get to pick PB number of specific spells that you can always transmute to your element free of cost?
I guess? My issue here is that if we go that far, that's giving the Dragon Sorcerer something like 6 free sorcery points per day
at level three. E.g., imagine a 4th-level character casts
scorching ray with all of his 2nd-level slots and
burning hands from all of his 1st-level slots, converting his 4 SP to two extra 1st level spells for more
burning hands uses. That's 4+2+2 = 8 sorcery points for free, at level
three. Sure, this is someone going HARDCORE for blasting (which is often inefficient), but that's nearly quadrupled (3->11) this character's number of spell points per day. Bump it up to 5th level, now we're looking at 4+3+2 = 9 free points at base, plus (in theory) another 3rd or one each 1st and 2nd for a total of 10-11 bonus points--and the number scales up indefinitely as the character gains more slots.
Picking specific spells doesn't (to my ears) change the impact at all. Even for a Sorcerer who spends half their slots on non-qualifying spells, that's still
at least doubling their number of SP/day, which is a very, very hefty power-up.
Perhaps, as an alternative, PB/SR? That way Sorcerers now have a reason to care about short rests (whereas before they simply didn't), and they can still
theoretically hit nearly what you're talking about if the party takes lots of short rests (which is good for short-rest classes like Fighter, Barbarian, Monk, or Warlock), but they need to be strategic about it between rests. As their PB grows, they can afford to transmute almost all spells for free, but could still feel a pinch if they went HAM in one fight and couldn't take a short rest before another strikes.
More or less, I still want there to be some tactical choices involved in whether or not one benefits from this. Just an always-on, "all your spells are <X element> now" is too much. Picking PB specific spells for it to apply to just means you'll nearly always have the effect in play. E.g.
sorcerous burst as a cantrip and
chromatic orb as a 1st-level (which Dragon Sorcerers get for free!) mean you never need to worry about not having enough to always push your "best damage spell of every level, always in my favorite element" button until
level 11.
With PB/SR free uses, that's 4-8 expected uses per day at 3rd, rising to 6-12 uses at 5th. (I am of course assuming every party takes a
minimum of one SR per LR; other classes pretty much
demand that this happen, so I think it's a safe assumption.) If your party skimps on short rests, you might need to dip into your actual SP now and then...but then again, you've made the tactical decision not to rest, so it seems reasonable that there should be a cost to that.