No experience with a fire dom, but I had an old plant/thorns which got deleted after I got sick of thorn redraw in a fight, and now he is reincarnated as a plant/psi, and I've been mucking about with fire blasters for a while.
AoE immob: This is fairly accurate, and you can use it to get out of the way of a boss by stacking with your other. Also utility with fire that if your foes are in a REALLY sweet position for fire breath, hold them still. I HATE it when they get out of the way, and instead of hitting 6 I time it too late and hit 2. But not a brilliant power, but I like it for hit and run tactics (run in, immob, mind probe the lot, get out, and repeat). I guess with fire this is droppable, but if you're running away it's great to fire off with the hold, you have a high chance of surviving to regroup and revenge.
Spirit tree: You have NO idea how many times this saved me. It turns you into a plant/etc/regen. Not sure what the heal stats of it are, but I remember the first time I used it in my first bank raid. The team I was in were attacked, and I thought we were dead. Called up spirit tree, and lobbed my AoE hold, retreated a bit while shooting off thorn attacks, and the corrupter was blasting everything that came near. Stalker was dead already (duh) and the MMs were being beaten. Killed some, regened up a bit, and then managed to survive the rest. No way would that have happened without that tree (ok, a brute would have done it better, but it's still a great power). It also, although situational, reduces the need of aid self.
Either drop concealment or, if you find yourself thinking "I'm completely screwed now" a lot, try picking up phase. It has been beaten senseless by the nerfbat, but is still a brilliant power in that situation. Phase, run. Unphase, regroup, and retaliate.
If you like hover/fly, up to you. I never use it on a plant because of concept (Plant powers need to be near the earth, same way you can't footstomp thin air). It is useful strategy wise, and since you hover you won't need CJ/acro for hold/immob/kb resist, since you don't mix up in the fighting much. I'd recommend hasten still, hasten+domination= God mode. Spamming holds and powers all at increased effectiveness= carnage.
You might want to go the way of assault/tactics for increased damage and accuracy, but they are end heavy, and if you're in mid air it wont be as much of a benefit to the team.
Never got a villain high enough for patrons, so I'll leave them to someone else, and hope that helped.
EDIT: typos galore.
I would back up what Seriphos is saying about Spirit Tree, it's not one I currently have in my build but when I tested the difference it makes is pretty big -
0->100% health 150 seconds (no tree)
0->100% health 50 seconds (with tree)
A minor point with Medicine Pool - I went for Stim not Aid Other since an anti-mezz power seems rarer than heals for villains. Hard to use though.
I considered concealment but I'm just too much up front in a fight for it to be that useful.
I'm curious about the Mako powers, I was very tempted by that set but have just gone for Scirocco, I couldn't resist the armour & power sink.
Wonder if I could garner some opinions here about respecing my Plant/Fire dominator. Shes sitting at a healthy L42 and does pretty well in teams, but does need some fine tuning. Ill list out what Ive got, what I like and what I dont like (no, I cant remember all the names of the specific powers, yes I am a total ch00b).
Plant
Has: Single Target Immobilise, Single Target Hold, Seeds of Confusion, Vines, Carrion Creepers, Venus Fly Trap.
I like all of these and will be keeping them. Ive no intention of taking the AOE Immobilize or Spirit Tree unless someone can give me a persuasive argument as why theyre not a useless aggro magnet for the former or just rubbish for the latter. I know Dom AOE holds arent outstanding, but it does give me a decent chance of holding a boss quickly into a fight.
Fire
Has: Flares, Fire Blast, Fire Breath, PBAoE, Blaze.
Im going to get rid of the PBAoE, I hardly ever use it. I use Fire Breath on a regular basis, and the three single targets are useful for cleaning up. I might consider Fiery Embrace in there, and/or maybe Blazing Bolt so Ive got some utility.
Pools
Has: Swift, Health, Stamina, Aid Other, Aid Self, Stealth, Invisibility, Hover, Fly
The fitness stuff is essential. The flight pool is my preferred travel power with this toon and gives me a good view for which mobs need holding/confusing etc. Aid self is handy and has saved my backside on more than one occasion. Im not sold on the concealment pool, so many things seem able to see through stealth these days and I really dont use invisibility enough to justify its inclusion.
Patron Pool
Has: Bile Spray.
I like this power apart from the obvious classiness of chucking your guts up on your enemies, its a second cone attack comparable to Fire Breath. This is, for me, a better option than the PBAoE in the fire pool.
So there we go, all suggestions welcomed. Normally Im gasping for slots to put powers in, here its more a case of finding something with decent synergy to go with what I have.