A few random thoughts for you:
Our ds was (and is) really into Legos. He transitioned easily into Minecraft - and my other kids (who like Legos but aren't passionate about them) also went through a phase of really liking Minecraft. BUT - fwiw, I wouldn't purposely choose to intro them to it - it's online, it's a screen activity, and for my gang, it was far more "addicting" than working on a Lego project ever was. So if you have any concerns about screen time, I wouldn't intro Minecraft until your kids first hear about it from their friends
If you're not worried about screen time, my kids also went through a Spore phase prior to Minecraft, and in some ways I like Spore better - it's something kids who love to build will enjoy - it's just a different type of "building" (you build civilizations). Personally I think it offers a bit more of a brainy challenge than Minecraft.
Re the saving things forever - my dd9 went through a long phase of that, but for her it was just a phase so it might be just a phase for your dd too. I don't have any great parenting advice to get you through it - for our dd we basically just had to say "these are family rules, we don't keep ____ (or we only keep ____ x #s etc)". I like the idea of taking pictures and keeping a scrapbook as a way of easing out of keeping everything.
Best wishes,
polarbear
