Shadow Knight said:
True. Sometimes you have to bend plausibility a bit and go for the excitement. In which case I encourage you to have Sicily go after Egypt. Forget poor Tunis go for the riches of the Nile, unexpected and could be quite the adventure.
If you want the Norman Med empre to become overextended and collapse, this is a good way to go.
However, if you want the Norman Med empire to stick around for awhile, have them go for Tunis instead. Once they add Tunisia to their empire and consolidate their gains, they could be strong enough to go after Egypt. Then you can have the adventure and (especially if the Almohades are still a threat), the unexpected, and the Norman empire can remain intact.
Tunisia is not entirely without its advantages. It's easily defensible and, unlike Egypt, isn't super-populated. It'd make a nice prize.
Plus, if there's a need for European settlement, the Normans could open it up to Lollards and Waldensians (as a reward for the Lollard spymaster, perchance?).
Of course, perhaps the Normans could launch a feint at Tunisia/Cyrenaica and then attack Egypt while the Egyptians and their allies have moved their armies west. However, they could use Tunisia as a stronghold and that could make finally getting rid of them tricky.