Honestly, I’d probably wait… At least til you have 12 tonics (or close to it), as imo, there are so many great green 5* out there in upcoming S4, Atlantis, costume portal, Valhalla, ToL and even Avalon (+ Lianna, maybe even normal Kadilen?!) that you could very well pull and max without regrets. Imo, with the heroes you have maxed already, I see no one that would significantly improve your roster enough to justify the 4* mats.
However, if chances are slim that you may pull anyone new in this element in the nearish future (ie you don’t do many pulls), and you really wanna max someone ASAP, may I suggest…
(Unpopular opinion here)
…
Telluria ?!
(I know, I know - I’m surprising even myself here with this one)
Okay, so why? Because I see absolutely zero mana control in your maxed 5* roster. & Imo, mana control - even a two turn mana gen slow like Telly’s - is too important to not have. Imo, it’s the (or one of the) most game-changing effects in this entire game.
+ Minions to pair with Freya
I obviously don’t love this option, but frankly, I think it’s your best. Her HOT will stack with Heimdall’s heal as well, so that you’ve got coverage all around (or another healer/support hero for war) + You get her ‘swiss army knife’ of abilities to deepen your roster a lil more (bound to be one you find useful, right? )
But to break down the rest of your options ~
My 2¢
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C. Elkanen really needs Evelyn to shine. His fast speed makes him pretty versatile for that alone, but overall, you’ll be disappointed with his damage output without someone around to buff his damage. Sure, he’ll be alright, but once you do pull Lianna (or literally any other offensive fast hitter), chances are he’ll get thrown on the shelf for good.
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Margaret is a contender for her tile damage, which you could make use of for titans and offensive stacks, but again, she’s got about mid-range use with a dodge damage for 3 (so really situational, as only dependable against snipers). Her hit at VF speed def boosts her from being mediocre, but it still leaves more to be desired.
So if it becomes between these two, I’d choose Margaret for titans, or if you stack heavy (ie 3-2 or mono), or Elkanen for his SS damage output
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C. Horghall/Bertila would both be great for VF rules. No doubt about it. C. Horghall would be more straight-forward, using him more for his attack ailment than anything else (solid for surviving against high-end titans), whereas Bertila would be more for her damage output - which is one of the highest you’ll see in the green element. However, her secondary effect is way too conditional to count on. & As you’ve said already, having Heimdall already maxed leaves little room for another slow mana hero in your team
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Elradir sucks. Please don’t.
Hope this helps
Edit:
Btw, if you do happen to have certain 4* maxed (and emblemed) like Hansel, Almur, Buddy, Marcel, etc, that could really sway the direction [of this decision] too, and be a solid deciding factor.
For example, if you do have Almur, then I’d go C. Elkanen no doubt. If you had Hansel (or even Peters), I’d be less inclined to suggest Telluria.