Vintage Paw - I posted a list of addons that I use a bit higher up the page, it dates from October but is still the list of what I use - in terms of essentials, Greymind's Quickslot Bar (allows you to hotkey potions/consumables) and Advanced Filters (for better Inventory management) are ones that I absolutely couldn't do without.
An awful lot has changed since you last played! I decided I really couldn't get along with Nightblades, there was just something about them that I didn't 'get' iykwim, but they are the class that is most likely to make me shit myself if I go up against them in PvP (fear, and high burst damage). So I can't help with specifics on their particular skills. Having said that, I think magicka Nightblades tend to be more effective than stamina builds, as most of their best class skills are magicka only (unlike say Templar, where many class skills can be morphed to a stamina version), and it leaves your entire stamina pool available for sneak, dodge, interrupt, and block.
A general tip for magicka builds in general, is that dual wielding maces with the sharpened trait actually makes all your offensive spells more effective (due to blunt and blade passive in DW line bonus for maces is to ignore a percentage of physical resistance per mace equipped, and sharpened trait increasing armor and spell penetration) - so it's worth considering having that set up on 1 bar, and whatever other weapon you like on the other bar.
For stamina builds, it's worth considering leveling 2H and having it on 1 bar because of the Momentum (morph to Rally when available) active, which is a stamina-based buff to weapon damage and a stamina based self-heal over time.
Alchemy and Enchanting - I would recommend leveling both, for starters both are a lot easier to level than they used to be. Alchemy - being able to make a variety of your own potions with 3 effects is invaluable at higher levels. Gathering the plants is the most painful part of the process, but it doesn't actually take that long at the alchemy bench to get to Alchemy level 50 (I had a big stock of plants and solvents and it didn't take me more than a couple of hours to max it out on 3 characters - and you're unlikely to need to make your own potions until you hit vet rank, so just collect plants and solvents for now and then level it when you need it). Enchanting - use Denata (blue) runes on 1 character to make glyphs with the highest level potency runes you can use, and decon them on another character (the one deconning will get most inspiration from the process, and again it only took me a couple of hours to max out 2 characters by that method). You will want to allocate skill points while leveling to use the highest level solvents/runes that you can, but can always respec afterwards (respec is now only 50g per skill point, so a lot cheaper and more viable than it used to be!)
Provisioning - either food or drink can be active at a time, not both (except xp booster drinks, which I don't imagine you will be using given that you are already overleveled!!) General rule of thumb is food for questing/PvE, drink for PvP where resource regen rate is more important. There are now a few recipes that do increase max of 1 resource and increase regen of another, they don't drop, they are Wrothgar quest rewards and many people prefer to use those for difficult content (trials etc.)
Crafting in general - because there are now so many style motifs and some of them are a pain in the arse to collect, if you are going to split crafting between 2 characters, have 1 do bs/cloth/ww and the other do prov/alch/ench - just so that you only have to get the style motifs on 1 character to have full use of them!
Argh, sorry about that, major case of verbal (well, typing anyway) diaorreah there, but I hope some of it is useful
