Let me also try to answer some of your questions:

what is this talk of some HDLC being rarer?
==> this is perhaps related to the item in the OHDLC that a regular (non-wiibrew) player cannot get because the slot in that player's town has been occupied by a different one (than what the player wants) since several OHDLCs share the same slot. As an example, my (non-wiibrew) friends cannot get guitar axe since they already have red lightsaber (*sigh*). They just need to be wiibrew players I guess.

when will more be released?
==> any good idea for this coming holiday/new-year season? I have created blue carnations HDLC this month for my friends and "all" players to celebrate Thanksgiving.

how can i make my own HDLC?
==> like DsPet said above, it could be tedious. Basically, you need to do open surgery of the codes in the game (to learn various things related to DLC part). It requires a great commitment to go through it. You need to learn few things: checksum for item, compression of file, 3D modeling and texture editing, setting behavior of item, and other little things. Have fun trying these. DsPet has in fact created a set of great tools that you could try first as the starting point. Hopefully, we could say to you "welcome to the club" (quoting DsPet).