Very few people looked at lego sets as a speculative investment play when GG retired (in 2010 (!)), which is why few copies were saved and prices for it are high now---demand is reasonable, but supply is low. GG also sat for a long time at 20% off on the shop-at-home website in Oct of that year, showing that demand was moderate for it, even then. 2018 might as well be a different world: more copies of modulars are being produced, many are being horded, and they don't last for weeks on sale. Depending on your time line and buy-in price, any set can successfully appreciate. I think the "buy it, forget about it, then cash it in" days for modulars are long past, however. Tread cautiously.