I have a lot of alts. I started the game intending to main my elementalist and play a guardian as well. Somewhere along the line--possibly during Caudecus Manor wipes--I swapped to levelling my guardian first. Both of them are now 80. Before I bought the expansion, I spent $10 on a character slot because I also wanted a mesmer, and possibly an open slot if mesmer wasn't to my liking. I was already planning out what might benefit the guild and what might interest me the most, and having only two character slots seemed a bit too restrictive.
After I bought the expansion, a decision that I made roughly five days after installing Guild Wars 2, I rolled a revenant, a necromancer, and an engineer. Warrior and thief simply did not look as interesting to me since I generally hate "hit it very hard" classes or "stealth around, be sneaky" classes in any game (and, on the flip side, elementalist and guardian are very me). Finally, because of raid comp and guild comp foibles that resulted in 75% of our players maining eles, I bought a character slot for a druid healer. I didn't want to delete anyone, and I also knew that, as GM and as a human being who likes to analyze video games, it was important for me to have every class eventually. I had planned to save thief and warrior for a long time down the road, but, thanks to a desire to parse as a warrior via GW2DPS, I bought an eighth slot for one.
I love my guardian and I'm working on ascended gear for him, so it made sense for me to have the other heavy armor classes, since you can use your ascended gear on any character that can use the armor or weapon type. However, revenant wasn't as interesting as I thought it would be, and warriors also use greatswords in their main pve spec.
Engineer pvp is one of the funnest things in the game--though I also like guardian pvp. Iolenn was doomed to a life of swimming in the Skyhammer mists until I realized just how much our guild needs condi damage.
Likewise, since I was interested in all three light armor classes, it made sense to try to level them up to gear-share. Because in the absence of a levelling plan I had just been hopping on whomever I felt like, I made a spreadsheet that looked at what type of gear was used assuming that your alts will be in a viable raiding spec--ie, something currently on metabattle or close to it.
My results were basically universal: have a set of berserker armor for each armor class, beginning with your main's armor, and then going to the tier in which you have the most likely alts--for me, light armor. With these power sets, you can tank by just adding a few pieces of toughness gear, or heal in full zerk or in an intermediate set. Just make sure your runes match!
The weapons that you can use with these power dps sets? Greatsword for warriors, guardians, and power dps reapers, and staff for eles, healing druids (you can get away with a zerk staff), thieves, and revs. A zerk sword may also be useful for several alts.
Condi sets and weapons are more specialized, unfortunately, and the condi mesmer build uses a different set than the condi necro builds do. However, you could probably get away with having a weapon with slightly less ideal stats, so I'd recommend building a condi scepter or pistol.
All of this should be adjusted for alts that you don't have, hate, or plan to sit in the Heart of the Mists or likewise, of course.