I want to suggest adjusting Stomping Ground or his ultimate:
Increase the scale of Stomping groundfrom 15%/15% to 20%/20%
Since it only procs on kills I see no reason why the scaling is up to 75%. With this adjustment players can use less points for an efficient reset High risk-High reward
Adjust Stomping Ground and give him an additional Elimination card like Evie (controversial)
My second idea regarding his cards is adjusting Stomping ground to only affect his Heroic Leap and to increase its scale to 20%/20% this would require it to get swapped with one of Bucks Heroic Leap cards. I am not quite sure which one. My only idea is the cooldown reduction card for Heroic leap to go and give like 1 second off with BH as compensation. This would mean to swap Stomping ground to replace Seismic Wave. On Stomping Ground's spot I would suggest a 50% elimination cdr card like Evie has. Buck is one of the flanks who does not have alot of mobility and gets outpoked easily by other DPS champions including Flanks. He is only effective at close range. Again the bigger scale on stomping ground would allow players to put less cards into it and still having an effective cdr for snowballs. Mobility is one of the last things being too strong on Buck.
Adjusting his ult to give CDR
This generally is something which I would love to see on Buck despite the changes above. To make him reset his cooldown with kills during the ultimate. His ultimate generally is not that great. Kills if one of the changes above would be considered as good. Elimination if none of the suggestions above are acceptable.
If anyone has any other suggestions regarding Buck feel free since I do not believe Buck is fine as he is as Evil Mojo has stated. He needs some love even if it is just a small Buff.