TorrentLeech has open registration several times per year. Keep an eye on Opentrackers.org for any of these. Note that some are open, while others are open application. The latter means you must meet certain criteria to be accepted. Typically this is proof of your stats on other trackers, but sometimes it's exclusively for refugees from one that failed.

Keep in mind that you will not ever find open registrations on an established, reputable tracker. They don't need more users. They only recruit from lesser, more accessible trackers. You will need to start on these to establish yourself. There are plenty of guides on this, with most starting on RED or MAM.

If you aren't on any of these, it's not because they're too hard to get into- it's because you don't want to put in the effort. Which is exactly what private trackers want to avoid.

Also, smaller doesn't always mean bad. TorrentDB was a rising star, with regular open invites, right up until its collapse. Even the giants like PTP started from nothing. Getting in early is a perfectly viable strategy, especially if you help grow it.