I like those ideas.
I think some very powerful spells should not be allowed to be cast in battle in real-time. I feel they should be prepared in advance. Some could be prepared 1-2 hours prior to you being able to use it. Upon casting the spell, which can only be prepared, a timer begins to count down. The spell points are slowly siphoned off of your maximum until the spell's magicka costs are reached. So if you have 100 Magicka but the spell cost 25, then your Maximum Magicka after preparing that spell will be 75 until the spell is actually cast and discharges (releases) that magicka back into your full pool.
Some spells of an even greater kind should take 6-8 hours in game, and some should take 24 hours in game. That would make the use of those spells extremely rare, and when they are used, they should be quite dramatic and awesome to behold. Make them worth the wait. And you know, this is only for a small few Greater Spells, not every spell, anyway. Most spells (90%) could be real-time cast spells and the rest could be Prepared spells.
In this way, the magicka would simulate being slower to regenerate for certain key spells, while allowing for the majority of spells to be cast frequently enough to make the game still fluid enough and fun enough not to lose the joy in it for those who use magick exclusively and need to use it fairly rapidly such as when looting dungeons.
Using this system, I dare say some more evolved battles later in the game would require some strategy as to how to prepare for the battle requiring days in advance to really "prepare" for the battle. That could really require some strategic thinking and planning, and not just run off half-cocked into the thick of battle like a meathead warrior. Mages do have to be tactical, and this would add to that immersion of that role, I think.
I don't agree with the planning thing, since I think that would take the life out of magic use, but the bold part I highlighted in your post is THE key issue. Magic should, at times, be absolutely dramatic and awesome to behold. To keep things balanced, this should come at a COST, and making away to limit the amount of awesome magic you can cast in a given time would be the solution to the problem. HOW we go about doing that is the real subject of debate for those of us who don't want magic to just be a different type of sword or a glorified projectile, but I like Achromatis' idea a great deal. Have our mage have to catch his breath or nearly faint after using his most powerful magic.
I always liked the idea perpetrated in the Eragon books with precious stones being able to store magickal energy. So, having a high quality ring or amulet with a stone, a mage would be able to store more additional magicka, while lower quality jewelry would hold less additional energy. Also, if you knew you weren't goning to be in a fight anytime soon, you could store your magicka away for future use.
All this could be implemented as a perk, instead of a default ability. Perhaps ability to store magicka could be related to Willpower, with lower levels only storing part of what they are absorbing.
Pretty good ideas. I think it would add some presentation elements is Achromatis' idea is incorporated as well: having the mage take deep breaths and such after a powerful spell, etc.