I think adding more variety to the sun bells, as well as buffing the companion bell (as mentioned in a previous thread) could make this choice much more meaningful than it currently is. I also think making some bells only come after the first or second boss could be interesting, since certain bell ideas (bounty, variety) would lose value the later in the game you get them. Additionally, this could help some builds that are particularly hard to enable become possible (charm).
Buffed Sun Bell of Fellowship: Add a character slot, immediately pick one of three (maybe four?) characters to add to your deck.
Sun Bell of Bounty: (something involving gold, and perhaps shop modification.) Such as: all enemies drop x more gold, shops are more likely to appear. Or maybe: All enemies drop x more gold, combos give x more gold, shops contain x more item(s). (Some mix of these ideas)
Sun Bell of Variety: Increases the amount of card options in everything by x. (I can see this one being too weak, and it definitely would suck after 2nd boss, but just an idea.)
Sun Bell of Constitution: All allies get +x HP.
Sun Bell of Charm: Choose one of three charms, x times. (probably twice)
Obviously some of these ideas are perhaps too strong or weak, but I figure this thread could be a landing spot for a bunch of sun bell ideas and balance changes.
Ideas added in the comments (for reading convenience):
A different Sun Bell of Fellowship (from Mixmaw): Add a companion slot, companions are immune to injuries. (Seems skewed in strength towards sac decks, but interesting)
Sun Bell of the Journey (from me):(A take on a comeback bell for someone whose deck feels behind) Add an extra stop (limited to treasure or a character selection) to every path.