Acorn/Oak is my favorite plant added to BfN, and as usual, a great concept with delightful voice acting, but the execution is lacking.
Sap Trap and Super Sap Trap actually used to work better before they patched it to reduce the instant vanquish super sap trap was causing; which was bad, but it's still doing that at times, and if they're going to fix that, Imp Punt also needs to be addressed, as it should have been back in GW or at least by GW2. They can still randomly injure other plants, do no damage to zombies, and other serious issues. I still contend that it should be a 'trap' instead of a direct fire bomb that most people use it for; give it an arm time and encourage it being used for actual traps or, like you point out, to send ourselves flying like the demoman or soldier in TF2 and some other games like Quake, it's so unreliable as it is right now.
Oak could be the plant's mech to counter the Imp, which is what I thought it was until we also got the team play idea and the Space Cadet/Station. I give the team credit for the idea of the acorn turning into an Oak and acting very much like a mech, and enjoy Oak a lot. The main issue with this idea is that the station benefits far more from other Cadets linking up than an Oak gets from treetop riding acorns, especially after they took the armor for being up there away, and you're pretty easy to snipe, while also taking full AoE damage. Being able to heal the Oak is nice, and they added overheal, but having shields, adding more health, and also having the generally effective torpedo and normal laser attack makes the station far more powerful, especially when full and overhealed! The station also lacks the activation timer that the Oak needs so they are out of whack, even if stations aren't the terrors they once were. I still love that they came up with the team play character idea, but I'd say to get rid of it for a new game and have Acorn/Oak be more of a direct opposite to Imp/Mechs, and drop the treetop riders altogether. While your treetop shot has no need to reload, it can overheat pretty quickly, and with sap trap being unreliable (and can fly at wonky angles if you're riding on an Oak), it's just not that great to be up there.
Acorn Dash getting a 2nd charge never bothered me, but didn't really see the need either, but it should likely be kept to balance them against Imps as they should be distinct and they lack the Imp's double jump. This is built on the idea of them making the classes direct opposites for the next shooter.