Just start expanding on your range of imperium. You can always make a mixed force army at 2000 points.
Get yourself some custodes- one box of five builds a patrol detachment of them. Two boxes and you have a battalion.
Grab a single knight box and add a super heavy auxiliary.
Oh, or maybe you like tanks and imperial guard is more you your liking- they have a huge range of dudes that do all sorts of things.
Combine that with some of your already existing marines and you could easily create a more dynamic list that is both ‘fluffy’ and viable.
More to the point is, in this current universe of 8th edition rules and army construction its advantageous and kinda cool to break away from static single force armies.
Obviously some factions can’t do that, and, I’ll admit it took me a while to start to accept soup lists in the meta but once I did it... it’s always fun to realize that you’re much less locked in on what toys you are collecting just to paint vs if you can also play them.