As a few have mentioned, if you stick with the 35mm, and you still have lenses to go with the SRT102, look into another Minolta body. Still a couple of my favorites in the classic Minolta camp are the XE7 and XD11. If you plan to do lots of bigger than 8x10 enlargements, med format is the way to go. Now, if you have glasses and getting worse at focussing the grandkids, autofocus comes to mind (I am debating on the autofocus at the moment) :)
I'll second Mongo's suggestions.
For a more portable system with good prices on great optics, consider the Pentax and Mamiya 645 line. I'd also pickup a used Minolta body as a second.
If you've got good Minolta glass, why not pick up either an X-700 or another SRT...There are tons of good bodies out there, and you wouldn't need to start collecting lenses from another company. Then you could go out and pick up an inexpensive MF camera to get a taste for it...
Just my $0.02
I would agree that if you want to stick with 35mm there is no reason not to stick with Monolta so that you can keep your existing lenses.
For medium format I have always taken the view that if you are going to do it you might as well go the whole way, so I use 6 x 7 rather than 6 x 4.5 (I don't really like the square format 6 x 6). For landscapes a range finder might be a good idea, but if you want to use an SLR, something rugged with a mirror lock up would be the best option. I have an RB67, but they don't like water and I tend to regard it as a studio camera. A Pentax 67 would work well. You might even consider skipping straight to large format.