Well, you'll get worse DPS with ice magic than you will with pretty much anything else, unfortunately. When the Morrowind expansion came out, people were really hoping for some good ice magic spells, but then Zenimax decided to make the whole skill tree into a tanking-only thing. Now the warden's ice magic skill-line is next to useless for most players unless you're specifically using it for a tank character. A whole lot of players were disappointed by that. (I like the frost cloak skill, though, and the frost ultimate is nice.)

One thing that helps spell damage, though, is to use a skill called Entrophy (from the Mage's Guild skill line) first before you cast any damage spells. It immediately raises your spell damage by 20%. A lot of people use the netch to do that, but I personally like Entrophy better than the netch, because the netch takes so long to summon. The only issue is that if your character is constantly short on magic sustain, you should use the netch instead.

The best way to DPS as a mag warden is to abandon frost magic. ZOS thinks of ice as a defensive element, and the Winter's Embrace tree is almost entirely defensive.

I've heard that some magwardens get good results with Scathing Mage, which makes sense, given that they have two huge sources of AOE direct damage (Shalks and Force Pulse). Nerieneth should actually pair well with that in the new update.

You don't. But I suspect they will be buffed, along with everything Frost.

You can do DPS as a Warden, your just not going to be pulling Meta like numbers. What you need to do is adjust focus to be a Survival DPS Warden. In that you use Class Skills to keep you alive while you whittle down the mobs. Now, despite what the Min/Maxers will tell you, it is a viable build. In fact I took down the Farm World Boss in Stormhaven as a Pug Duo at L14 with an L6 scrub that was hanging out doing what they could.

Now, granted it is an easy boss to kite, but still, it's a world boss. What allowed me to do this was the Healing I could do on my character, and of course the spastic moving around that was required. Several times during that fight I was healing myself back up from like 10 Health.

So, you may not be all that successful in a Vet Dungeon/Trial, but I have seen worse than my Warden prevail in some of those. But most any other PvE content can be done on a Mag Warden as a DPS/Heals.

The other thing you have to do that goes against the DPS Meta is set up your attacks as most of them are either DoT's or have some set up time, like the Cliff Racer one. So fights will take tactics rather than YOLO Spam of skills until they are dead like you can do with a MagSorcerer.