I think you are heading in the right direction as well. If you want time pressure, you need to add an aspect of a countdown or time limit. Deus Ex: Human Revolution has a similar concept to your ideas about a network trace. The hacking was a mini-game that you could undertake to open doors, safes etc. without having to search for the key or wait for the story to advance to the point of making the key available.
A hacking attempt starts out with the system unaware of your presence. You are 'jacked into' the terminal node of a network diagram. The goal is to subvert nodes along the graph to reach all of the server nodes on the graph. Each action you take has a time to complete and a risk of alerting the system. When you trip an alert, a network trace starts from server nodes and if it reaches your terminal before you complete the graph or voluntarily disconnect, then you fail and face the consequences. The trace was essentially a countdown timer, which made each hacking node suspenseful on its own and made the entire thing a race against the clock once started. Consequences were usually just having to wait for the system to reset, but occasionally an alarm would sound and guards with guns would come try to kill you.
You can watch the in-game tutorial for the hacking system. There were various upgrades you could buy with your hard-earned experience points to improve your hacking abilities, as well as some single-use items you could find that would help with difficult hacks.
The mini-game was fairly interesting to learn and a vast improvement over 'hacking' in the first two games of the series, but it became tedious by the end of the story line. If you spent enough points on upgrades, most systems became trivially easy to hack, while the hard ones remained very difficult to complete fully. If you didn't spend your points on hacking upgrades, hard systems remained nigh impossible to take down without blowing through stacks of the worm software items.
Also, as there were only a handful of node-types to overcome and only 4 specific actions you could take at any given time. Every hacking attempt would devolve into either throwing all your items at it or just attacking every node as quickly as possible to get to the end.
In particular, if you added more ways to prepare for and react to the network trace, you could make your game much more enjoyable than Deus Ex's version.
Adding more mechanics of time pressure could end up being too similar. One idea that springs to mind is that the player must maintain the integrity of their connection to the system. Taking some actions decreases this integrity, taking other actions might hold it steady or even bolster it. If the integrity reaches zero, the signal is lost and your hacking attempt ends.
From a broader perspective, you can look at time as a resource the player must manage. Adding other resource pressures can increase the intensity of the time resource. You could try limiting the number of actions the player can take (the actions themselves become a resource) or the rate at which they can be taken (having to wait for some focus or brainpower resource to recharge between actions).
Riffing on Phillip's ideas, you could simulate the resources of the attacker's machine. Launching one kind of hack takes up x% CPU and yMB RAM for z seconds, or running a trace evasion program takes up a chunk of resources continuously. You can only take additional actions when your machine has the CPU and memory available for it.
You could also add an aspect of timing to the user's actions. Make it so certain actions can only be taken or completed at certain times. Or make it so you have to click, hold and release buttons at specific intervals to finesse your actions. (Like sports games frequently do: click & hold for a power level and click at the right moment for timing. Here's is a field-goal kicking game that implements this mechanic.) The better the players timing, the more likely their action is to succeed, or the better the outcome of the action.
Finally, if you want to reduce the starkness of the network trace - where either you're getting counted down or not - you could implement a heat or suspicion activity. Think of Grand Theft Auto or Thief or Assassin's Creed, where doing suspicious or illegal things raises the attention of the guards. If you keep being bad, the guard will attack or pursue you. But if you actively do something to deflect their attention (hide in a shadow, duck down an alley way, blend into a crowd, etc.), they leave you alone. You could adapt this mechanic for an anti-intrusion system without much of a stretch.
Good luck and it sounds like you're onto a fun game!