I am trying to plan on a second language to learn after Romanian. I want it to be something useful (spoken a lot) but also quite similar to Romanian, so that I can make connections.
Any suggestions would be greatly appreciated!
Well you could pick pretty much any Romance language (French, Spanish, Italian, Portugeuse) as they're related. You could do a Slavic language too but I believe Romanian vocabulary is mainly Romance and the grammar is more Slavic-like.
The grammar of Romanian is, in fact, not similar at all to that the Slavic languages.
I kinda meant to say it's Slavic like as in it has its original Latin cases unlike the other Romance languages.
I haven't done enough Romanian to be sure, but I think it's more that Romanian is a Romance language that has grammar closer to Latin than the other Romance languages and has some influences (and vocab?) from Slavic languages. I don't think it's particularly Slavic in grammar.
(IIRC it does share some features with the Slavic languages of the region, including some Slavic ones, from neighbouring countries, as part of the Balkan sprachbund, but that is a long way from saying it has Slavic grammar. After all, it also includes, afaik, Greek and Albanian, not just Slavic languages.
I'm not sure if you would want to learn this language at all, but somebody began a discussion by telling that Dutch is really fun to learn at duolingo!
Go with Italian or Spanish. They are the closest languages to Romanian and the easiest to learn for us Romanian speakers.