Title says it all - my personal list of suggestions for making this feature usable while also keeping the world economy the same:
1) A challenge, maybe Sundays only, should allow players to accumulate Shard Shop currency. Make Tier 3 drop 100 credits per clear, so that a max-level player only accumulates 300 currency per week.
2) All characters should be available all the time. You can still only buy shards for them 1/day with no way to refresh. It's outrageous, given that it would take MONTHS to farm a character from 0-7* in the shard shop, to also add an element of praying they show up in your shipments to the list.
--As a compromise to the above, if they really aren't feeling it, at least make Vader + Chromium-only heroes permanently available here with a rotation of farmable heroes as well.
These are modest changes and would still make it a lengthy process for a F2P player to get a usable Darth Maul or get their Vader to 7*, but currently it's so cumbersome as to be 100% useless to F2P players while ALSO being miserable and time-consuming for P2P players. With these changes, a P2P player could more quickly accumulate what they want with their massive currency, while F2P players could either scrimp and save for months and months to get a chromium exclusive, or better actually use the shop to accelerate their farming for tough-to-farm heroes like Rey, Ima Gun-di, Barriss, and Dooku.