The absolute least number of buffs Cap needs to be a reliable B-tier pick

[Nomad]

This post is going to reference the current Thor team up ability multiple times as TTU for short

(New Primary) Sentinel Toss
-40 Damage, 50 when upgraded (In line with TTU)
-Stock system replaced with a dedicated shield throw primary on a 2nd weapon, similar to Punisher's rifle and shotgun
-No ammo system
-Canceling Sentinel Toss with Living Legend or Leading Dash no longer removes the shield projectile from the world, even if this means multiple copies of Cap's shield existing at once
-Landed hits now restore 25 health to shield life bar

Sentinel Strike
-Rather than stock system forcing Sentinel Strike to be a ranged attack after 2 punches, it now upgrades Sentinel Toss to TTU's attack for four charges
-Stock/Upgrade system no longer has a 5 second time limit on usage before being lost
-Landing another two Sentinel Strikes fully refreshes Upgraded Sentinel Toss stocks if stocks remain
-Options in Cap's controls allow for automatic switching to Sentinel Toss when upgrade requirements are met, or requiring a manual switch
-Landed hits now restore 25 health to shield life bar

Reasoning: The main strength of Cap is supposed to be adaptability for forcing the character to use a ranged attack is literally the opposite of that. What if I want to continually pressure a shield tank from the front? The character's kit shouldn't force a change in strategy, the reality of the match should. Also, if the stock system is to remain, it should produce an actual benefit in damage and function, not force a side-grade.

Vibranium Energy Saw
-Canceling the move with Living Legend or Liberty Rush no longer removes the shield projectile from the world, even if this means multiple copies of Cap's shield existing at once
-20% reduction in damage per enemy hit removed

Reasoning: this is Cap's one good move for damage bursting and its pretty lackluster for 70 damage. Allowing him to cancel into other moves without sacrificing the scarce amount of team damage it causes adds to that whole adaptability idea.

Living Legend
-Reflected projectiles now follow the direction of Cap's targeting reticle in a cone comparable to Psylocke's primary fire
-Cooldown removed or reduced to .3 seconds if its necessary for some reason

Fearless Leap
-New animation added for Cap raising his shield during fearless leap
-Apex of Leap's height slightly raised so he can jump to 2nd story platform height frequently used in map design

Super Soldier Slam
-Cooldown reduced to 8 seconds
-Hitbox increased to be slightly larger than targeting symbol

Liberty Rush
-Displacement effect of running into enemies when combining Living Legend and Freedom Charge now a standard effect of Liberty Rush
-Immediately grants upgraded Sentinel Toss stocks on hit, since you're knocking your target away
-Cooldown reduced to 8 seconds
-Can now be aimed in any direction, including upwards
-Hitbox increased to be slightly larger

Reasoning: Further adding to Cap's adaptability. LL and FL changes are self explanatory, SSS and LR should not be treated as simple escape tools which otherwise deserve high CD values. Cap being able to put his shield in the right place at the right time during a team fight should be his entire gameplan, NOT diving the back line which is what players currently do to try and create value with the character. Liberty Rush in general was an extremely weak move with very little vision behind it. This now gives Cap some of his displacement potential during ult on his hotbar and allows him to use it to quickly ascend and reach places normally out of reach by using Leap alone.

Freedom Charge
-For the duration of Freedom Charge, Living Legend can be used with infinite shield bar health
-For the duration of Freedom Charge, all uses of Sentinel Toss are upgraded

Reasoning: Cap needing to use his mobility to make sure his entire team is benefiting from Freedom Charge's bonus health and mobility is smart and in line with the character, but having no effect on his damage or shielding potential at all is unnecessarily limited. Now he can displace the enemy, buff his team, and slightly raise his sustained damage as needed.

Final thoughts: The main way I think Cap could be further improved is to slightly work in Cap's bonus health generating potential into his kit beyond his ult, particularly if its done in a way that incentivizes diving to and from team mates strategically. If, for example, Super Soldier Slam generated a small amount of bonus health (25-50) to teammates in range on landing, it would add to the idea of leaping in just in the nick of time to save a healer from a dive dps, but then cool down reductions would probably have to be reevaluated. I tried to be fair and focus on small QoL improvements that add up to a big buff overall. More uses for tools, more adaptability and fewer limitations, giving the enemy team more of a reason to target Cap before he's the last one alive, and giving him more ways to counter that targeting, along with more access to his shield during team fights.