From the wiki:
Battlemages (also known as Warlocks) get into the middle of the fray, seeking to wreak havoc upon the entire enemy team with their overwhelming sustained area damage. Due to their relatively short (but not melee) combat ranges and the need to burn down their opponents over time, Battle Mages have significant defensive capabilities that range from sustaining endlessly to literally defying death for a short period of time.
So, if we don't want to add more damage into the game, what else might battlemages find useful?
Movespeed (Phase rush) - yeah, sure, but as already pointed out it's not much use to a mage who's stuck in position, or whose kit already stops/slows the enemy, or who is waiting for his cds after proccing it.
Some defensive ability?
Might do... not a straight defence boost, no, that would make it mandatory on tanks since there isn't an actually full-defensive keystone.
Sustain then? Heal for a base+scaling value over X seconds after casting a spell; this should reward spell spamming in a fight.
What else could battlemages use?
Let's see... they all seem to have some CC.
Maybe an effect that weakens the enemy after the CC ends?
Grounding sounds good, but even just a slow or a slight reduction to that enemy's damage might work.
To prevent this being used to 1v1 people they're not supposed to, it could also scale with the number of champions (allied or enemy) present in the area.
What else?
Don't know, can't think of anything else right now.
P.S. I'm a little offended that Fiddlesticks is not categorized as a battle mage.