I am an Eladrin elf who would like to be able to misty step into the bag of holding and back out of it. I was wondering if it's at all possible to do and if the bag would need to be open for me to do so. The rules do not say anything about interdimensional stepping.
The answer to this question can't be found directly in the RAW, so we must engage in some speculation, specifically the success of the action hinges on what you see when you look into a Bag of Holding and what you see when you look out of it. Let's look at the descriptions:
Briefly surrounded by silvery mist, you teleport up to 30 feet to an unoccupied space that you can see. (PHB pg. 260)
Bag of Holding:
This bag has an interior space considerably larger than its outside dimensions, roughly 2 feet in diameter at the mouth and 4 feet deep. The bag can hold up to 500 pounds, not exceeding a volume of 64 cubic feet. The bag weighs 15 pounds, regardless of its contents.... Breathing creatures inside the bag can survive up to a number of minutes equal to 10 divided by the number of creatures (minimum 1 minute), after which time they begin to suffocate... (DMG pgs. 153-154).
As you can see there are several requirements that must be met for you to able to accomplish this action:
You must not put the weight of the items inside the bag to over 500 lbs, meaning that you must add your character weight, items you are carrying, and items already inside the bag to see if you exceed the weight limit (if you do the bag breaks and its contents are set adrift in the astral plane!).
You must not stay in the bag for more that 10 minutes (assuming your the only one in there) lest you suffocate.
As per Misty Step you must be able to see where you are going- which is where the speculation begins.
Since you have to see where you are going, you will not be able to teleport into or out of a closed bag, but if the bag is open and you can look inside you might be able to justify the this action (similarly, if you can see out of the bag when inside). So basically what it comes down to is the description of what you see when you look inside (or outside) the bag. Since no official source (to my knowledge) contains such a description it would be the DM's call. If you can see a tiny storage room with all your items or even a sea of floating items you could probably justify using misty step, but if you see blackness, a void, or even just an empty bag you can't use Misty Step.
In conclusion - Maybe, it depends on whether or not you can see into or out of the open bag's item storage area (I'm hesitant to say that it is an extradimensional space b/c it is not specifically stated in the RAW) which is dependent on how your DM describes the bag. However, even if your DM excepts this justification, it may be much easier for you (depending on your size) just jump in normally.
Jumping in shouldn't be a problem, you can see the space (or you couldn't get your stuff). Misty step just wants you to be able to see the space.
Once inside you get in, since its got a volume of 64 feet, you could end up farther than you can misty step. More importantly you have to be able to see the space you want to get back to. Remember that from the inside you are in a large lightless weightless space, looking thru a max 2 ft hole to see a space. I would rule that all you can see is light, making this suddenly an interesting adventure for our multidimensional misty stepper. So now what? The only spell I can think of to get you out is dimension door (plane shift might work too).
This does explain the abundance of well preserved bodies found in recovered bags of holding...