>>712482863Truck kun is old news. The new hotness is dying because of overwork/black company related deaths.
You are right about portals, as summoning would probably fall into that category, and its also fairly popular.
And the crawl space linked to another world... you guessed it, also quite popular, however this one is popular with stories where the main character is meant to go back and forth.
The only ones I have seen very rarely (maybe once or twice) would be the painting/book you mentioned. Even the dimensional merge isn't anything new, however since its the shonen genre, its usually the classroom that gets merged with another world, or there was another one where the real world and the otherworld started overlapping, however that is less isekai and more fantasy.