there's been a recent thread from someone asking some of your same questions, have a look
http://boards.cityofheroes.com/showthread.php?t=287308
1) personally i like the ATO set in roots, including the ATO proc, and a Grav Anchor chance for hold.
2) having the achilles heel proc in everything is redundant, since it does not stack from the same caster. if you have it in more than one power there's more chances for it to hit overall, but once one of them hits, that's it. in a defense focused build you probably wont have room for more than 1 anyway, Fling Thorns is a good spot.
3) imho permadom should be your first goal, and defense second; having said that, permadom and 32.5% to S/L shouldn't be hard to get if you are using scorpion shield.
So, I have a currently level 43 Plant/Thorn Dominator(first one) and she has been quite a blast to play, and I'm considering shelling out inf for a build for her.
I have a few questions before I go into mids and try to work something out
1. How do you all propose the slotting of Roots? Damage? Immobilize? Set bonuses?
2. For /Thorn attacks, which attacks would be ideal for throwing in Achilles Heel procs, and how many attacks should hold the proc? Would all of them be redundant? Would 1-2 of them be not enough?
3. What is a general number for defense I should aspire for on my dominator. Would 32.5% be a good goal, with the rest of the bonuses aiming towards recharge bonuses primarily to aim for permadom?
Thanks for all and any input.