Pro meta is a mixture of unbalanced heroes like Sniper/Troll, and FotM heroes like SF/Storm/Lina. Your question doesn't make sense because it's implied that you don't want Prophet to be unbalanced, you want him to be FotM. But pro FotM is kind of senseless; nobody predicted Elder Titan's or Axe's rise, and everyone thought LC was going to be a 90% pick/ban mid but she proved to be extremely clunky in CM. If Prophet were shoehorned into the current meta it would also probably not fit a later meta, so we wouldn't have a long term solution to our Prophet problem anyway. Instead I'll just propose changes to his role.
If I reworked Prophet my objective would be to encourage Prophet to abandon his team in order to split push. The counterplay against this hero would involve 4-5 manning down a single lane with midgame push heroes or sustain.
Jungling Prophet already sabotages your lanes, because you'll have a super poor 5 position, and he still vacuums farm off the map with his ult and split push. He also leaves those lanes vulnerable to very early pushes and aggression. I'd want to emphasize this trade-off: picking the reworked Prophet would mean that you trade farm on your position 1 and 5 for farm on your split pusher.
A playstyle that I've wanted to test out on Brood/Tinker is to draw 2 enemy creep waves into the jungle using Treants then divert the enemy creep waves to enemy jungle camps, thereby causing 2 enemy lanes to cut at once and getting a lot of farm for yourself. You'd need to give enemy players a deterrent to hunting you down in the jungle though, e.g. +2 armor/+2 regen/second on Prophet and Treants while near a tree. But then Prophet would be just a weaker form of Broodmother.
If I wanted to make Prophet stronger in the offlane I might replace Sprout with an ability that sort of combined Bristleback/Undying/Razor, giving him a passive that causes him to eject Treants whenever he takes x damage OR is the target of single-target spells (only one, haven't decided which). This would give Prophet a way to deter the enemy team from spamming spells on him because soon they'd be chest deep in Treants.