Regaining those days is a noble goal. The increased reliance in the hobby on official and "complete" rules and settings has diminished the hobby as a creative outlet. That said, some more modern games, viewed correctly, provide more of that vibe than we in the OSR acknowledge. In particular there are two systems readily available (one in print and one out of print but very, very common) that provide what I'll call RPG legos. There is so much published material for these systems, much of it contridictory and some of it trash, that a GM can easily mix and match his way to a unique game completely distinct from the DM next door while still being able to draw on fans of those systems. They are the Palladium Megaversal system and, wait for it, D&D 3.x/D20.
Palladium, in particular, is very much old school at its core having began as a house ruled version of D&D. Several games with different genres or settings exist or have existed. Checking the Palladium website there are currently in print twelve game lines using the Palladium core system (more or less) and 116 individual items in print or back-ordered. They also have several prior lines now out of print but readily found. The design of Palladium's books make them perfect for a mix or match campaign (as well as great OSR supplements). Each book is a mish-mash of new rules, classes, spells, monsters, and items (magical and tech) with some setting pieces. Even these bits of setting are often easily adaptable to a variety of settings. With new Gygaxian building blocks in each sourcebook you could pick one core rules set and then just pick two more books that wet your appetitate (a la Jeff Rient's Alchemical Formula) to create an interesting campaign. As new ones tickle your fancy or you need new material you can just add a book.
Before moving on from Palladium let me add one final advantage and a caveat. The advantage is Palladium material is often found used if your FLGS has a used section. It is also quite common new or used on eBay for very reasonable prices. The caveat is the Palladium system can be a bit unweildy, especially if using the MDC rules. Combat requires a lot of judgment callsand nothing is consistant across systems. For an OSR game this problem, as well as many other complaints about Palladium's system such as a dozen different classes for soldiers or skills having all different percentages to give, are pure gold. In addition the web abounds with houserules as does Palladium's The Rifter or you can roll your own.
D&D 3.x/D20 (henceforth D&D3 for simplicity) is a tighter system with less room for judgment calls so beloved of the OSR but more than we grognards credit it. At this point it has at least several major variations six or seven by my count) at least two of which have their own minor variations. They cover fantasy (too numerous to list), science fiction at least twice (Star Wars and Traveller), espinage (Spycraft), superheroes (Mutants and Masterminds), and generic modern/historical/future (D20 Modern). Beyond that there exist different tweeks for other styles and conventions (i.e. BESM D20 and True20). Add in a ton of mostly compatible D20 branded supplemental material and you have more than enough to build a unique rules set. More over, much as Palladium, most of this is now in the used market due to 4th edition.
D&D3 has three advantages over Palladium in practical and artistic terms. First, finding players willing to play is easier as the system is more common and familiar to a broad base of players. Next, there is more material available, although almost to the point of being overwhelming. The final benefit is the material is OGL. You can, to a large degree, compose your own player's handbook out of all the pieces you've selected and distribute it. You may have to rewrite lots of descriptions as most OGL materials are OGL only in the game related materials and names while descriptions are considered PI. However, in my opinion this is a plus not a minus. Allowing you to combine the best of OGL game material with your world specific descriptions allows you to create something new and unique to you but clearly built from the widest available game system.
