Having Spectrals once every WL map for a period of 48 hours is in my opinion a good addition to the game.
Being able to upgrade your Avatar gear with crystals(fires increase damage, greens boost health) and being able to make your own mix of stats to your weapons and armour. Unlocking a slot on a piece of gear would then cost certain things, experience, credits, platinum or a mixture of the afore mentioned.
Armour expansion: Again a rather simple expansion with effect on a large scale. Every piece of armour currently in Legacy available will get a matching pair of gloves/gauntlets, a helmet, greaves, legs, boots and of course the currently availble cuirass/chest. This would create some depth to the item system and it open up a credit sink making levelling possibly a longer process. It'd also work well with the afore mentioned upgrading through use of crystals and combinations of crystals. More pieces of gear means more upgrade slots to unlock and thus more varietion in stats.
Expanding the current set of combat stat enhancing abilities(Target practise etc.) with another 5 levels, these could all take as long as the current level 5(+-50 days) this would give avatars something to train and create a bigger gap between starting avatars and the more seasoned ones. It'd grant a bonus for those who stick around. In light of this idea we might be better off with Ability point weekends rather than double exp. weekends. In these Ability point weekends any kill made, be it in PvP, WL or in hunting, will have you rewarded with an amount of ability points calculated by the amount of Experience gained.
The first idea isn't particulary small but it does build on our current item system, coming up with something that would link sertain combinations of crystals to the unique item code of your item shouldn't be too complex. The armour expension would simply add to our current item system which should be extremely easy, the only things that needs to be re-designed are profiles and inventories.
And last but not least, the expansion of our combat abilities by another 5 levels per ability would also be adding on to an already existant system.