It says you can trade XP for a new ability, but can you ever up the dice to represent someone getting better at a skill? would make sense - and if so is there a limit on how many dice can be used for a skill? EG trade XP for an additional dice in a skill or shift the dice up.
Yes, to trade XP for an additional die in a skill or shift up you'd simply select the appropriate ability with your XP. For example, if you wanted your Ally to be a better shooter you could pick 'Marksman' to boost the number of dice he rolls. This would cost 2 XP.
Remember not to duplicate an ability on the same character, but if you look around you can find something that's close to the same thing.
... a method where perhaps a follower has lasted a couple of games, with enough XP could you turn them into an ally (or start the progress)
There is no game rule/mechanic that will turn a Follower into an Ally. However, players are free to remake/reorganize their leagues as they progress. For example, Mila recently gained an extra roster slot due to a Reputation bonus, so she decided to remake her Follower as an Ally.
I'd also like to think perhaps a league would grow to include a few other people at leader/sidekick level but you may not be able to field them in the same game.
The only real constant is the Leader. Even Sidekicks can be switched out between games. I've seen players make several completely different rosters around the same lead character.
For example, for one scenario maybe you decide to team up your Leader with his old girlfriend and an Egyptian guide. After a few games maybe you decide to swap his supporting characters for a nightclub singer and a short Asian boy. After a few more games, you decide to team him up with his father and bring the Egyptian guide back. And so on...
I hope that helps.
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