Hmmm, I don't know that I'd actually like or be able to write something by myself due to my comic reading habits, mostly picking up trades of stuff I hear is worth my time, If I could ask HXS continuous continuity questions I'd probably want to bring back X Man back and pull Nate Grey off of his higher plane of existence, Emma Frost and the rest of the earth bound psychics thought Psylocke being back at full power and in her orignial body was a shock to the system, Nate's return would put them all in frickin comas.
There was always allot more they could have done with him as a character in my mind, and with all of the newer and returned villians around, Sisterhood of mutants, Weapon Omega, Huber, etc... You could throw allot at the guy and keep it a fairly action packed title while fleshing his character out more than it was and reworking allot of his views and characteristics after being one with the entire universe would be a fun challenge in my mind.
That and since the 616 draft a few years back on the WUMB I've had it in my mind pretty much constantly that Threnody really needs to get used by some title due to the sheer awesomeness of her powers and the potential chaos she could cause in any books she appeared in again, and with her connection to Nate it would be the most sensible place to put her. (For those unaware, she's an energy vampire, can create Spiderman strong Zombies, and can use the power of any mutant that has died even if they've been resurrected.)
Other than that I'd go with the majority on here, Exiles would be a blast to write, all those alternate versions of characters at your disposal, all the worlds available, you could buy a complete set of What If's and go to town from there.
Guess that would be the last one I'd write, What If...? Taking all of the major events and changing around one detail and running with what could have happened instead... That series would be a blast two write, and being able to ask the input of the original writers of things they would have liked to see instead of what they were directed to do in some cases would make it an easy yet rewarding title to work on from everyone's point of view, mine, the original writers, and the fans.
In all honesty though, unless I had access to a whole lot of source material and knowledge from a source like HXS or other Marvel experts I probably wouldn't be all that great at pulling off a comic on a regular basis, if left to my own devices I'd probably pull a Grant Morrison and just do what was cool in my mind and consequences be damned, I'm looking at you Final Crisis..., which would ultimately lead to having the complete and utter contempt of the entire Marvel fanbase even if they didn't read the books I was working on.