DammitVictor
Trust the Fungus
As an alternative, i have used multiclass as a feat. Spend a feat to get the ability to multiclass into a single class, the basic training.
I have designed a number of alternative multiclassing systems for 3.X over the years. Unfortunately, I cannot claim that any of them have been any good at all.
I've usually preferred variants of Gestalt with LA or XP penalties, or a spending a feat to take a second class and then getting class bonuses based on character level. The latter has some promise, but I never got it right; I think using 5e and Pathfinder's Variant Multiclassing as source material might be promising.
My multiclassing system for Sellswords & Godwars is based on reducing occupation/ancestral archetypes (mostly) to feat lists. Take a multiclass or hybrid feat, you're eligible to take ancestral/occupational feats from the selected archetype. Bang. Done. Unless you're a Human or a Bard, you're only allowed one additional occupation, and only Humans are allowed to take hybrid feats at all.