The problem with making things up on the fly is it feels arbitrary and unfair to the players, or simply work plain poorly for not being coherently thought out beforehand. Most creatures have a specific set of powers because they are synergistic. For example Dragons have a powerful MBA, which deals a significant amount of damage with their bite but isn't usually their first choice of standard action on their turn. The bite becomes the dragons out of turn opportunity attack for the most part, while their double attack is their go to power. With a creature like a Lich, I would be seriously thinking towards powers that can manipulate PCs own magic and turn it against them. For example, being able to turn an enemies zone or conjuration into an unfriendly zone/conjuration. This has the advantage of making the monster more adaptable to circumstances as well.
A well designed solo shouldn't need to have infinite powers to show a high degree of interesting stuff. It should have enough for the 3-4 rounds that it will last in combat (ideally) to be more than interesting - without turning into a huge long grind like pre-MM solos. As such you should only need to make around 4 maybe 5 standard action powers to accomplish a great deal of variety. I would aim for 4 standards, 2 minors, 2 triggered and a couple of traits. That's a big and very complicated monster right there - but it will certainly have the options to give the PCs the feeling of being a very challenging (and adept) spell caster.
Even those abilities can work very well with one another, such as a vulnerability granting power that interacts with an energy type the Lich can do (at its very simplest), then that is even better again. Such things should be thought of ahead of time and simply making them up on the spot is not a good solution. Especially because - in my case anyway - my initial solo designs are always far more over the top than need be. I design by making something ridiculous and scaling it down. I then like to "playtest" my solo against the actual players that will face it - just to fine tune it and see if it actually works correctly. 9/10 this method has produced fun, challenging and generally interesting solos. If there is a monster in 4E that deserves to have such effort put into it, the solo is certainly it.