Lefty Rickenbackers? Seriously, I have no idea; and neither does anybody else, really. Some people will guess right, of course, and make some money, but that's what it will be; a shrewd, but lucky, guess. My opinion, for what it's worth, would be:
!. High quality basses; not necessarily boutique, though. People generally seem to want things they've heard of. Some boutique basses, though would qualify; especially if they are -
2. Basses used by players that people have heard of. Who will they be in the next 30 years? Sorry; no idea (but I'm an old f**t).
3. Basses actually owned by those players, of course. Key there is iron-clad, verifiable provenance.
4. Basses that are scarce/rare now: they will be even more so 30 years from now. But again, not necessarily boutique. Rare colors, like Rick's recent SnoGlos, or their Color Of The Year instruments (if people only ordered 25, that's all there ever will be); limited runs, like Gibson's recent unannounced leftys; stuff like that.
5. Leftys. Lefty Rickenbackers, for example, do actually fit most of the above criteria. People have heard of them; at least one current famous bassists plays one; they are at least perceived as a high quality instruments (that debate is for another time and place); and they are certainly scarce. I know; I have 2, and I paid a lot more than they cost originally for both of them. But, if you corner the market, please don't make me pay too much more than I already have for the next one!