For clarity, I will now elaborate on the 4 possible ways to implement Fortify Health:
1) Vanilla Morrowind: This is the old method of fortifying your current health, and only your current health. After the duration wears off, you lose the fortified health. This was very dangerous, and most people agree that it was rather pointless because it was an inescapable death trap that not even healing could save you from.
2) Morrowind Code Patch "Fortify Maximum Health": This is the current method. This functions as a health buffer that allows you go into battle with more total health, allowing you to take more punishment for a duration. The benefit is increasing your total amount of damage that can be taken in a set amount of time without healing, and increasing the effectiveness of healing in larger amounts. The downsides to this method are that in its current form, it isn't very useful when compared with other spells currently in the game. The cost ratio of Fortify Health when compared with spell such as Restore Health and Shield or Resist Magic are much more cost effective, having better performance ratios. In order to increase the usefulness of this form of the spell, you need to implement changes to all of Morrowind, increasing the durations of all of the items in the game, and decreasing the costs of creating custom spells and items, and making potions of this effect last longer. So in conclusion, in my opinion, I believe that this way is not practical to implement in anyway, and the current state of affairs is not useful enough to warrant casting the spell.
3) Temporary HP: This method would treat Fortify Health as a preemptive heal. That is, the hitpoints that it adds would be removed first when damage is taken, and then, when the effect wore off, any remaining temporary health, if it remained, would be removed. The downside to this method, however, is that it creates an exploit with constant effect items containing the effect, allowing a person to constantly heal themselves by reequiping the item constantly.
Edit:
After some forethought, it could be possible to balance the "Elaura" method. You could simply have constant effect Fortify Health act as it does now, simply give a boost to Maximum HP and take it away when removed, and give temporary ones the proposed change of acting like a preemptive heal.
4) Percentage Based Fortify Health: This is the method that I recommended to Hrnchamd because of practicality in implementation. This method contains no exploits, and allows for the Fortify Health effect to useful for tanking, and it scales down as health increases, causing it to be more useful for weakling mages in the beginning.
Please consider these options before voting, and if you have anything say, please post and let everyone know your thoughts.