So for the past few years since about ~2013 I've been on and off keeping an eye on Michael Stonebraker and his work on VoltDB (based on lessons learned from h-store) and Scidb and in general his criticisms of nosql and newsql variants.

I think Scidb, being a column-oriented dbms geared for multi-dimensional arrays (datacubes) is very interesting given current trends, and there are only a handful of similar dbs around, the other two that interest me are rasdaman and monetdb. I don't know if OmniScidb counts as a datacube db but it is also really interesting especially due to it's gpu and caching model.

As a sysadmin/ops type having to deal with monitoring timeseries db's are also something I like to keep an eye on. It used to be mostly rrdtool in this space, but now I am comparing prometheus and influxdb. Like OmniSci, another case of something that's not quite in the same db model but might even better a better solution for the space (metrics) is Apache Druid. (elasticsearch being another than can be massaged to fit as a quasi tsdb as well) I think there is some room to unify the monitoring/metrics and log storage arena into one space (usually they are separated which adds admin overhead) and right now I really like druid as a potential for this.

Another interesting application of timeseriesdb's that I have been keeping an eye on is in the quant/algo trading area. Most people have been using kdb+ there but many are looking for replacements and there are some really good conversations to be found about the kind of limitations they are hitting.

I'm just a sysadmin who likes to keep up with whats going on, and my db knowledge is limited, but I do have a process for narrowing my focus to the dbs I reference. It must be open source, bonus points to gpl or apache licenses. The language it is written in is important but not a deal breaker (very tired of so many java based dbs). I don't like it when they are tacked on top of an "older" tech (such as kairos on cassandra, timeseriesdb on top of postgres, opentdsb on top of hbase, kudu on hadoop, etc). Being either filesystem aware or agnostic can be nice features (playing well with ceph, lustre, etc) Not saying this is the sort of selection criteria others should use just giving some info on mine.

A few more interesting mentions: clickhouse, gnocchi, marketstore, Atlas (Netflix), opentick (on top of foundationdb).