I´m really glad that you dropped that concept. I also think that it wasn´t really fitting to B<.
The imo bad part about it was that it would have forced you to play on other worlds for the sake of getting to a higher tier, even if you´d have really liked the world you are currently on.
Sticking to the word “tier” as indicator of progression I´d say that every world should have several (all) tiers the game offers. A world map with that system could, for example, look like this:
With the difficulty increasing from green (where new players spawn and the capital is located) to red (where creatures are scaled up to a point where they can only be beaten by cooperating groups of players). This should, of course, be a gradually change without strict “borders”.
To incorporate titans into this system I think that they should follow certain paths (like the black spiral) and not randomly float around. Following the spiral the titan would spawn in the redzone of a world where it is the hardest to beat but also gives the best loot and then start to slowly follow its predefined path towards the center. During this journey the titan becomes continuously weaker (runs out of fuel etc.) but also the loot gets worse.
This could lead to a pretty fun race for the best loot as you would have to decide to engage the titan now and risk to fail or wait a bit until it gets easier but risk that a better group might raid it before you have the chance to.