Given that Piercing Thorns is on level with Colossus Slayer, the addition of Guardian Soul on top of that is too much power creep for my tastes. While in tree form the ranger could use also large weapons too, so Piercing Thorns could be dropped all together. Plus, Ancient Fortitude and Guardian Aura seem stronger than Defensive Tactics or Superior Hunter's Defense so some balancing is in order.
Piercing thorns is worse than Colossus Slayer.
Thorns is 1d6 once a turn
Slayer is 1d8 once a turn, slayer does have the limit that the enemy has to already be damaged, but once you are 5th level and have 2 attacks that limit drops off significantly. I think it is a limit that will come up very rarely.
Guardian Soul in addition to that would be more powerful if we were looking at lv 3 exclusively, but every other trick the ranger gets is predicated upon being in Guardian form for it to work, and Guardian only having 5ft of movement is a large penalty to have to work around I think. Not impossible, but you will notice it.
Oversized weapons are regulated to a per table basis, as some of the discussion on this thread has already shown.
Ancient Fortitude has been brought to once per short or long rest, for those of us chosen to receive the update (I wish I knew why it wouldn't work for me, but going forward I will assume that is the case)
Guardian aura is very, very strong, but so is getting Evasion or Uncanny dodge
The Warden Ranger probably edges out the hunter as being slightly better, but I wouldn't think it is by very much.