Horde Rush!

 With the announcement of 9.2.5, Alliance guilds everywhere are experiencing a horror show! Alliance toons are jumping ship to get a slight benefit in raiding. Many are simply faction swapping their current high-level toon, with a horde race of the same toon. 

Void Elf Enervati

=>

Orc Elkagorasa

Fortunately, there are so many advances to leveling up a toon! Using Chromie Time, players can choose what expansion they want to level in. A lower max level cap means only half the number of levels to quest. Queueing for dungeons is less painful as it's possible to run a dungeon with 10-50 level characters, not just a tight band (ie level 40-50) of players. 

What class?

1. Pick a class you want to play. If you're spinning up a new character, maybe it's time to try out something new. While starting warlock over again will mean you know the rotation, Warcraft has a number of options that might also be fun. 
2. If having a hard time between two classes, try the multi-spec option. Classes that can both tank/heal and DPS give a fairly decent speed boost as dungeon queues pop a LOT faster for tanks and healers than they do for DPS.

Gear / Heirlooms:

Heirloom gear, once a mandatory staple of any altoholic, has become slightly less important with the level squish. It no longer gives bonus XP based on the number of items. Even though, I still invest heavily in heirloom gear for any of my new alts. I just spent 60k gold upgrading my existing mail heirlooms to max level.

1. Heirlooms scale while leveling. If you upgrade them completely, the same gear piece can be used at level 1 as level 59. 
2. XMog is always awesome! Since the major pieces of gear never change, (weapon and off-hand, head, chest, legs, back), the majority of the transmog stays in place. If you blank out the remaining (non-heirloom) slots it's free to hide again. 
3. Enchant once and forget. Going back to WoD, there are enchants available for most gear slots. Enchanting heirlooms is a possibility with most enchants, with the note that some slots can't be used. For example, current expansion enchants can be level locked, like Eternal Stats is locked to 50+. Whereas Glorious Stats, is not useful on items OVER level 50. Enchant now and forget about them until starting Shadowlands content!
4. More rested. As the rested bonus is consumed slower, continue questing longer.
5. Always decent gear. Heirloom gear provides the feeling of always having cutting-edge gear. Happily pull larger than normal packs of trash so that I can finish a quest chain slightly faster. 

Macro to display how much rested you have saved up:

/script p="player";x=UnitXP(p);m=UnitXPMax(p);r=GetXPExhaustion();if -1==(r or -1)then t="No rest."else t="Rest: "..(math.floor(20*r/m+0.5)).." /30 bbls ("if r+x<m then t=t..r else t=t.."level +"..(r+x-m)end t=t.."XP)"end;DEFAULT_CHAT_FRAME:AddMessage(t)

Questing

I suggest installing the add-on Azeroth Auto Pilot. This free add-on will give you the best route through the last 4 expansions. The authors love WoD as it has a ton of treasures that give a decent bit of xp simply for locating them. Following the little breadcrumbs between each quest objective, you'll level pretty fast. As of writing, I am ~3hrs to get to level 22. 

Once the garrison is upgraded to rank 2, you can build the storehouse and you have bank access. 

Scouting Maps:

Purchasable toys from vendors that will provide most of the taxi stations in each continent. I'd suggest at least the current expansion 

Shadowlands alt-friendly options:

content skips:

• Torghast intro, land directly in Oribos to decide upon threads of fate or storyline. 
• Renown skip - purchase a "0-60 renown" token from vendor by flight master. I hit 80 soon after getting to max level. 
• Run back to back torghast. While leveling during my early 50s, I was getting almost a full level with each run, plus a level of renown for completing the daily quest. 
• At max level you'll get a series of options that will allow skipping a good portion of content. 
• Korthia zone skip -  never visit the zone
• Zereth Mortis zone intro skip 

Purchasable armor:

•  Zerith Mortis has vendor selling entry level quest greens. 228 for 500g each. Replace your lowest items, but don't over purchase. 
• World boss and quest rewards was 233 and 246 gear. 
• Sandworn relics are BOA, you can mail them from established mains to alts for 246 items.
• Purchasable item at Friendly with ZM faction Enlightened for 246 gear (shaman got boots). 
• Purchasable BOA 'pouch' of soul ash and soul cinders from established Korthia base vendor (at 10% loss). 
• Purchasable BOA Cosmic flux pouch from ZM quartermaster. Transfer roughly 4000 cosmic flux if you can to purchase max level legendary. 3000 for the pouch of everything needed to make legendary (ash, cinders) and 1000 flux left over for the 291 ilevel item. 
• Free 260 legendary for completing all 6 chapters in ZM, 3 are completed with the ZM zone skip.