aigleborgne

I really wanted to have some random creatures to match PnP, so I went with this:

7th: Dryad (55%), Hamadryad (30%), 5HD Treant (15%)
10th: Hamadryad (55%), 5HD Treant (30%), 7HD Treant (15%)
13th: 7HD Treant (55%), 9HD Treant (30%), 11HD Treant (15%)
16th+: 9HD Treant (55%), 11HD Treant (45%)

It is kinda like Elemental Summoning so you can have something stronger with some luck. 
I might balance it more at higher levels as I am not totally convinced it should be stronger. Spell Revision did it, but so, every summoning spells should improve with levels?

Maybe it could improve a bit up to 7HD Treant but more seems overpowered for a level 4 spell.

Yes, but then, she would serve as an uncontrollable ally which it is not convenient. Anyway hamadryad doesn't provide much more than a dryad. You will mostly use their charms. They have better defense though.

For now, I have added 5hd very young treant.

I'd like to make a random call 1d2 dryads (love your suggestion) or 5hd treant.
As for treant, I need to compare it with animals summoning of the same level. It is 2 leopards with SR. They probably have more offense than a single treant but the later has many immunities due to its plant trait.

I need to be cautious to keep it balanced while remaining truthful to PnP.

I have tried 1d4 dryads and I think it might be OP as well.
Dryads dire charm is powerful (-3 save) and they can cast it 3 times. So it is potentially 12 dire charms if you are lucky to get 4 dryads.

I could also create a very young treant with lower stats and about 5-6hd.