Hi and welcome here (I think I didn't welcome you before).
For shell ideas I don't have any right now, but make sure we can do the essential like unlocking/archiving programs from it. Also try to make it not too large and not too slow. Note that this will be hard for a BASIC shell to compete against Doors CS7 or even MirageOS, though, especially that BASIC shells historically had a very bad reputation on ticalc.org, but it can be a good way to practice some TI-BASIC/Celtic programming.