Because 5e did away with stat PENALTIES, and because AdvLeag uses the standard array (15,14,13,12,10,8), and stats hard cap at 20, it's been my observation that ALL class, race, background combos are viable.
So, what I like to do is make a randomizer chart. All my race options, all my class options, all the background options, listed out and numbered.
Then I roll like three or four times on each list, and create some idea stubs.
For example, getting ready for a con in November, I decided to roll up three characters using the Volo races.
I rolled Tabaxi-Soldier-Warlock, Lizardman-Criminal-Cleric, and Firbolg-Acolyte-Fighter.
The Lizardman tickled my fancy, so I built him out and created a backstory.