I know it's risky trying to be That Guy With The Ideas in a thread like this, but how about.....
- New York seemed to be an awfully convenient outlet for ghosts in two movies' worth of in-universe time...
- So now, it's been 20+ years and it's happening elsewhere in the world. (Explanation? not much required, just that it took the forces-at-work longer to find another outlet, far from the containment vault in NYC to be on the safe side.)
- Paranormal experts know who to call, but the GB crew are scattered to the winds since NYC. All acknowledge they're older and might not be able to handle it; Venkman is off on permanent vacation in the caribbean or something.
- Ray and Egon are genuinely jazzed, scientifically, to be part of a new happening. Winston, maybe not so much. He always seemed like the reluctant one.
- They need more hands, more minds, so they turn to a couple trusted acquaintances, or maybe even interested volunteers, to round out a new team. A blend of passing the torch and hauling their old selves out again; definitely better handled than a simple 'new generation' approach.
Could work...