Look for an Existing Solution, and Respec/Educate
Here's the example Xivort power discussed in the question:
Net (weapon) At-Will DDI
Attack: Area burst 1 within 5 (creatures in the burst); +5 vs. Reflex
Hit: The target is restrained (save ends).
Here is a abridged similar feat (there are several):
You catch an enemy in your net and maneuver it into position.
Attack: Your highest ability vs. AC
Hit: 1[W] damage, and the target is grabbed. The grab automatically ends at the start of your next turn. If you used this power as a melee attack, you slide the target 1 square. If you used this power as a ranged attack, you pull the target 1 square.
So the first thing I always do when players want to do something, is find out how closely I can get with the existing rules. The Compendium makes this easy, if it is possible at all.
Here's how my conversations go in situations like these:
"OK, so you've collected the nets, but aren't trained in using them. There are feats and/or powers that would give you similar abilities, but you'll have to give up one of your other feats/powers in exchange - you haven't been using that action-point-related feat - would you like to switch it for Net Snare instead?"
This way the player sees how the new rules attempt to integrate common player desires (such as wielding a net to ensnare) with game balance.
Usually, this approach works well. Sometimes I need to reskin a feat/power to fit the exact situation (for example changing a pike-based power/feat to work with a lance....)
But, when there's just no way that the 4e rule set is going to stay "balanced" but I really want a player to have something, I ...
... Risk Breaking the Game to Fix it
I've given out Wings of Flying to my 3rd level 4e Artificer, because it made sense with the story and the character. But - Rules As Written - low level flying powers are very, very, limited (typically encounter or daily powers with only turn-length limited use) - not good enough for me.
So, I gave her a much more advanced item, with powers similar to level 10 and stated that she could risk repairing and improving the item, giving it hover and longer time aloft - which she invested every hour and gold into during a month long break between adventures and succeeded at the all-or-nothing skill challenge.
Similarly, I gave the rogue another character a backpack (Power) much more valuable than any Bag of Holding:
Wait! I have one of those! - Daily, Minor Action
“Hey! I know I picked one of those up back there... at that place... you know... Yes! Here it is!”
Effect: You quickly search through your ample backpack, pockets, around in the current area to find a specific mundane, non-weapon item.
These items can be used to break adventures, as written. Yet, I never regretted this decision. I love it when they use these to break encounters.
But, I then use that so-called "unbalance" as license to restore balance to the encounter by adding complications. The flying wings allowed them to assault the Scepter Tower of Spellguard more easily, but - after the BBG engaged the party once, he returned with his own special flying ability in the final battle, hampering the rest of the party's participation. [The artificer ended up carrying the rogue up so she could Backstab the flying villain...]
Experienced GMs adapt encounters to the situation at hand: how powerful the party is, what their character histories contain, what their opponents have learned, etc.
There is no pure mechanical balance - the computational balance is only a starting place that the situation and participants (players, characters, and DM) distort as much as possible to tell an interesting encounter-story.
Once I embraced my Old-School (1e) ways and embraced upping the threat-level whenever my characters are getting too uppity, I've been a happy 4e DM.
[Note: In practice I do both the above almost every week - try to find/reskin existing stuff and just go ahead an break it - and fix it as I go along...]