In a game where a single snowball can spell the doom of a player clad in full diamond armor and an inventory full of notch apples, I really don't think a strength potion is the biggest issue when it comes to balance. If a player uses a strength potion, simply kite that player until the potion's duration has run out or adapt your tactic to fight that player at range.
Skywars is a strategy-based PvP game, so if someone's strategy is to literally hulk out, you can counter that by forcing them into a situation where they can be spleefed, knocked off, shot, or lured into a trap. Stay mobile, don't allow yourself to fall into a siege situation, and remember that the alteration of terrain can be your biggest ally. For further reading, check out Chapter 10 of Sun Tzu's The Art of War.
P.S. "When you engage in actual fighting, if victory is long in coming, then men's weapons will grow dull and their ardor will be damped. If you lay siege to a town [or Skywars island], you will exhaust your strength [potion.]" -Sun Tzu if he was a gamer.