It's probably fair to say that the X-Men have had more spinoff series than any other comic in history. (It certainly seems like every past, present or future member of the team has had his or her own series at some point.) One of the more interesting X-teams to spin off of the main series was made up of some old-time favorite characters, a few new faces, and even (gasp!) a couple of non-mutants. This was the Great Britain-based team, Excalibur.
Originally written by Chris Claremont (with outstanding pencils by Alan Davis), the book was largely responsible for introducing the one element which had been missing from the other X-titles: fun. Although Excalibur still shared the same four-color sensibilities as their cousins across the pond, their off-beat adventures and unusual cast of supporting characters made for a whimsical, tongue-in-cheek twist on Marvel's increasingly serious mutant franchise.
Possibly as a side effect of this lighter writing style, the book introduced TG aspects to the stories with surprising regularity, although they never really took center stage. Having a shapeshifter on the team certainly helped, but the heroes encountered enough body-swapping villains and transdimensional alternate realities to make some degree of gender changing almost a foregone conclusion.
The one recurring TG thread that popped up occasionally involved the luckless and scheming Nigel Frobisher. A minor supporting character, Nigel was in many ways the whipping boy of the series, constantly bouncing from one misfortune to another. In one memorable sequence, Nigel was briefly transformed into the image of his female employer Courtney Ross (among other things). However, his real introduction to femininity occurred when he teamed up with Captain Britain's reality-warping brother, Jamie Braddock, who arranged it so Nigel could take on the form of the female underworld crime boss, the Vixen. Unfortunately, the Vixen is pretty well past her prime, but it still makes for some of the more interesting TG moments of the series.
Early in the series, Kitty Pryde (Shadowcat) goes undercover as Phoenix to prove that she can take care of herself. In keeping with the spirit of the series, she gets captured almost immediately. She runs afoul of the Warwolves, a group of strange creatures who "eat" humans so they can take their skins and impersonate them. After getting eaten by one of the wolves, we find that her phasing power has unusual effects on her attacker.
After Arcade captures Captain Britain's ex-girlfriend Courtney Ross, Excalibur races to the rescue. Unfortunately, the Crazy Gang is ready for them, and uses a mind-swapping device to thoroughly confuse matters.
Excalibur visited so many alternate realities and dimensions it was almost unusual to see them on Earth for any length of time. During the course of the "Cross-Time Caper" story arc the team sometimes visited several alternate Earths in the same issue, each one more bizarre than the one before. Generally they were limited to brief references, as shown here.
The team's resident shapeshifter and Captain Britain's on-and-off girlfriend, Meggan provided for some interesting TG moments, both for herself and others. At one point during the team's escapades through alternate dimensions, she and Phoenix manage to get their powers crossed, so Phoenix finds herself shapeshifting against her will to Meggan's thoughts. To fight off Jamie Braddock, Meggan even takes on the appearance (and powers) of all of the X-Men making for an extended series of transformations. (The first and only F2M2M2F2M2F2M2F transformation in comics, I believe. )