We don't know yet how Forcecage will work - if it allows a save, the Fighter will be able to deal with, if it doesn't, WotC screw up and Monte Cook is secretly still working for WotC, boosting the spellcasters.
I really don't want the Fighter to be flying without magic - but I like the idea of him jumping extreme heights and distance - the abilities are already there. The trick is to word these sensibly, I think.
Maybe I should have a new ability for that.
I'm Gonna Ride the Dragon (Level 6)
As the Dragon turns on his wings for another attack with his breath weapon, you start running and jump onto him...
Spend one stamina to make a jump 4 times as wide and high as normal, and make a melee attack roll with advantage against an opponent within range (including a flying opponent). If you hit, you do not deal damage but instead have latched onto the target. If it's your size or smaller, it will fall down with you and cannot fly until you let go. If it's larger than you, it's flight speed is reduced in half. You are considered grabbing the creature and have advantage for all checks to avoid being thrown off until the end of your next turn. You can make normal melee attacks against the target. Whenever you would fail a check to keep on the target, you can immediately spend one point of stamina to keep latching on and gain advantage on all checks doing so for another round.
[Note: The ability is explicitly formulated giving advantages on this activity - this gives leeway to allow this maneuver to everyone, not just fighters with this ability. But the Fighter will have an easier time.]
This may be interesting.
Spellbreaker (Level 9)
You notice the enemy's protective spells - this isn't the first time you've seen such in action, and you learned a few tricks to deal with them.
When you hit an opponent that is protected or affected by benefital magical effects with your melee attack, you can spend 3 stamina to suppress the effect. If there are multiple effects at work, choose one effect if you know the specific effect, or the DM chooses the strongest effect affecting the target. The effect remains suppressed until you spend a round neither adjacent to the target nor attacking it.
[NOTE: If a DM finds it inappropriate in his campaign that the fighter can counter magic without using magic himself, you may require that the fighter has a magical item, for example a magical weapon -for this purpose.]
If you're willing to spend 7 stamina every day for a long-term favor, sure, you can do that. The important part is that the FIghter has advantage on any checks involving this negotation. Of course the DM can be a d*ck about it and never let the party roll or decide between 1,000,000 or 2,000,000 astral diamonds - but really, do we need rules to cover d*cks? If we had that, we'd need rules in 4E like "A buzzing fly does not impede on a short or extended rest, even if DM d*ckward would like it to be so."
Also, the target can always choose to live with the death mark and risk the fighter having the ability to kill him at any time.
The most abusive way to use this would probably be to threaten a party member in eternal servitude, forever giving him advantage on all ability checks. But hey, that's 7 stamina each day for one ally getting advantage. I think it's about as far as a trade off as casting Extended Bull Strength, Cat's Grace, Endurance, Eagle's Splender, Owl's Wisdom and Fox Cunning on an ally in 3E. Except probably even more costly than in 3E, since 3E did give wizards much more spells per day than any edition before (and quite possibly any edition afterwards...)
I probably should settle down on one ability score. We'll see.