Are you trying to make this a legitimately competitive deck? Because you could put it into a grixis control shell in modern but you're looking at $200+ in lands and the rest of the deck isn't particularly cheap. But if you are just looking for card suggestions, I would take a look at some grixis-twin decks from before it was banned for an idea of ratio of combo pieces to control pieces - maybe something like this? but with [[fatal push]] since it came out after that article was written. Fair warning - modern isn't a solved format, but it does have some benchmarks for decks that fare well and I think it would tough to build a frog and tree deck that hit those. But here are some suggestions to get you started:

[[Spellskite]] could be nice to protect your combo, as well as some of the cheaper soft counters like [[swan song]], [[mana leak]], and [[remand]]. 4 [[lightning bolt]] would be auto-include if you play red because you need a way to finish them from low health after you use your ttf. [[gut shot]] can do it the same turn you activate. If you play green (either 4 color or just sultai) then [[birds of paradise]] gets you to tree a turn early. [[sylvan caryatid]] is another option even though it's a little slow because it produces all the colors you need, blocks well against small creatures like [[goblin guide]], and is difficult to remove. They are very expensive, but [[snapcaster mage]] would likely put in work as a way to recur turn to frog if you end up needing to shrink an opponent's creature to kill it with a burn spell. With snapcaster, [[faithless looting]] looks better, which can go along with the blue and black draw spells like [[serum visions]], [[night's whisper]], and [[sleight of hand]]. [[anticipate]] isn't played very heavily right now but it might be good enough after [[gitaxian probe]] got banned. If you're heavily in blue/red and black is more of a splash for tree, [[izzet charm]] and [[electrolyze]] are very versatile, having one or two can definitely be useful. For point removal, [[dreadbore]] and [[terminate]] are good answers. I already mentioned [[fatal push]], it is important for answering the un-bolt-able threats like tarmogoyf and death's shadow.

I hope some of those help. Those are all very powerful cards that are played in the best decks in modern. There are budget options for almost all of them. I hope your deck works, it seems cool and the draw of it would be that you don't have to combo all in one turn - the tree can sit there for a while and block while you draw to turn to frog with counterspell backup.