Bumblebuncher if you can get hold of the booklet Bernina Accessories - solutions for your ideas, available from Bogod's Cardiff warehouse (their phone number is 029 2079 2079) and inside the front cover of this are groups of Bernina machines by their numbers, and are grouped by A,B,C,D,E,F,G,H. So for your existing machine the Activa 135S you come under the letter B. Then all the pages that show the different feet and accessories have given the groups that the feet are suitable for. You would find this extremely helpful I am sure, and then you could tell which machine you can use these existing feet and accessories for. This booklet is free or it was when they sent me my copy, as like you I have collected a huge amount of feet and accessories for my two Berninas. I have an old 830 Record(1975) and that is not mentioned in this book, and then I have a much newer one an Artista 200. Now the feet for the 830 Record do not fit on the Artista 200, nor do the bobbins and a lot of the accessories. By the way my Artista comes under Category G.
Over the years as you probably know Bernina have brought out a few huge machines with enormous bobbins and feet which are totally different in the shank to what we are used to, so if you can get hold of one of these books I am sure it will help you and guide you to what you will go looking at. By the way you are being sensible in trying to find something that you will still be able to use your existing collection of feet with.
Incidentally when I got my latest copy of this book, it came with a separate paper retail price list. As a matter of interest over the last year or so Bernina have been doing something called foot or accessory per month, and they select which one they are offering at a slightly reduced price. Bernina (Bogod) have their own direct website now and if you use this the orders go straight down to Cardiff.
A few years ago now when I bought my Artista 200 it came with what they called a Trolley or Suitcase System - my machine does not have a hard cover, instead it came with a huge zippered bag on the trolley. The Embroidery module came in a huge cardboard box - and yes there was a matching bag to house this but was an optional extra. Well where I intended to store the embroidery module would not take this without it getting damaged, so I then spoke to a lady who had run the Sewing School at Bogod in London for a very long time - she has now retired. Bernina had also discontinued the style of the matching bags, and of course I wanted the embroidery module bag to be the matching one to my main machine bag. Well bless this lady within the day she had come back to me and had found such a bag, and of course this was tucked away in an obscure room down in Cardiff, and I received this much reduced in price the very next day - that was how I really got to know the ladies who operate down in Cardiff and prefer to use them when I am buying feet or accessories as I am dealing directly with them - and paying the same as I would from a dealer, but I do not have to keep on going miles to visit a dealer and then back again to collect, they come in the post usually the next day.
If you do get in touch with the Cardiff place, ask if they can send you The Complete Catelogue for this year and that should list all the currents models and what they can do.
Good luck with your search and I do hope you can find something that suits you and that you like.