Which Dino(s) should I upgrade next and what order should I upgrade them in?:
should it be velociraptor and/or T-REX or instead level up INDOMINUS rex?
I’d go Stegodeos, Irex, then raptor
Stegodeus and Stegoceratops should be priority. Very biased wth Indominus but since you seem to have some rexy pts leftover, and have already committed, i’d just aim towards lvl 20. Stack your pts on velociraptor If you are planning on working towards the indoraptor. Level up your veloci once you get your Irex to 20. Raptors in the arenas are pretty awful to use now, even at level 20
Don’t level up the nodo, and keep it at 15 for stegodeus.
Piggybacking on what said, Stegodeus and Stegoceratops and my top 2. My Stegodeus is level 20 and it is a boss. As a rule I only level legendary hybrid ingredients to level 15, unless I know it’ll be a while before I can create the legendary. For example, level your amargocephalus to 15, then replace it with tragodistis (and you’re already on the right track with a 15 parasaurolophus). My one exception to the rule is stegocertops. I like it better than monostegotops, which I’ve created but haven’t leveled past 16.
Keep your raptors high level, second priority goes to your tanks. Level common Stego as much as you can - for a common, it’s worth it! Try to get your rex up a couple more levels if you can. It’s great for anti-armor. Same goes for Allosaurus as well. I used it in battle once and it made a 2000+ hp crit on the opponent.