If you have enough endurance, defense and recharge, and since you are not one of a handful of characters with enough resistances to benefit from the res alphas, the default choice is Musculature. It improves the damage of judgement and lore pets as well - I found it really nice to be able to judgement for 700-800 post-debuffs and have cim pets that hit for 1k+.
I might consider agility if taking it allowed the build to maintain the softcap while dropping weave or powerboost (presuming you're relying on one or the other to softcap), or if it allowed the build to come close to the incarnate softcap.
Yea, I think Musculature is it. Depending on which route you take, you're looking at either a moderate boost to your damage and utility or a hefty bonus to damage, which is perfect for Fire/Time. It's almost like Musculature was designed with this combo in mind.
The problem with Cardiac for range is that your best buffs and debuffs are PBAoE's, meaning unlike other secondaries you really want to be closer to the action so your team can benefit from Times Juncture, Farsight, and Chrono Shift. Musculature's bonus to Endurance Modification makes the Endurance Reduction less of a trade up, so the only thing that it really nets you is Damage Resistance. Overall Cardiac has a heavy opportunity cost for something you probably don't want to use much anyways. I wouldn't mess with it.
My Fire/Time doesn't feel like it's lacking anything. This is a relatively cheap build (no purples/pvp ios) with mostly level 40 enhancements so I can exemp a ways down without losing all of my bonuses. It does have perma-hasten & chronoshift and ranged+aoe softcap. I'm effectively softcapped to melee with Time's Juncture. I'm not building for any specific content; I'll be running iTrials, TFs of various levels, general pug teams, etc.
I'm leaning towards Musculature because damage is yummy. More recharge really isn't going to help; I'm not having end issues, but my blue bar isn't impervious so Musc's endmod is nice. I could go Cardiac, but I really don't need that kind of endred. Cardiac's range would be nice, but unecessary. Although Musc would be a small damage boost it will probably provide more benefit.
Can anyone think of a reason to go with something else?