So I played and loved Dynasty Warriors 2, 3 and 4. I own Dynasty Warriors 8 and I'm loving this as well. Is it worth it for me to revisit any of the DW games that I've missed in between? DW5-7, Strikeforce, any of the Empires games? Are any of them worth a play nowadays?
1. 5 was basically a DW4.5 in a way (i.e. few new characters and every character had their own story mode, similar art style to DW4), similarly sort of how I view 8 as DW7.5
I think 5 would be tough getting back into after playing DW8, but that's just my two cents. It can probably be found for cheap now if you want to give it a try but I found it myself to be not that big of a gap over DW4. I still enjoyed it years ago though.
Also, some people including me are of the opinion that DW5: Empires is the best of all the Empires games. Mostly because you actually have control over where you on the map you can specifically place officers and stuff, there is a lot more micromanagement compared to 6: Empires. it is the closest you will get to ROTK in a DW title I would believe.
2. I would honestly just avoid DW6 and 6: Empires at all costs really, many people (including myself) view it as the lowest point in the series with characters being cut, no new ones being added, not everybody having their own story mode, and shitty Renbu system for combat.
3. DW7; I think is worth playing for the story/musou mode alone (which I personally put on par with DW8's story mode thus far). The gameplay in DW8 is basically an improvement in what Koei started trying out in DW7 with EX weapons and the such, and it can also probably be found for sub $20 nowadays. Having put over 100 hours into it, I can recommend.
7: Xtreme Legends can be easily skipped (it added 3 characters that are in DW8 anyways), 7: Empires I heard is good, but I don't know if it is worth paying $50 for now.
Summary: Play 5 and 5: Empires if you want, but it will definitely be harder to go back to after playing DW8. Skip everything that has to do with 6 (except for strikeforce maybe, that game was fun). Play DW7 for the story mode if you want to, you might as well skip 7: Xtreme Legends at this point and take a look at 7: Empires to see if it is for you.