There are various issues with your ideas inthe context of doing them as an FPGA. For the webbrowser accelerator, it sounds like you are suggesting a processor that executes human-readable code, as I do not think that HTML5, javascript etc. are "compiled" binary executables. And creating a parser in hardware rather than in software. My view of processors is that these things are better done in software. Perhaps what makes more sense is an FPGA accelerator, something like Majsta's project, or perhaps something like a Zync chip using the ARM cores to emulate 68K faster, or as a fast coprocessor like PPC started out on Phase5 boards.
I see the mouse/joystick ports as way way too slow to do these sorts of things. Go CPU slot/socket or go home. As others have suggested, you may end up duplicating the FPGAReplay as an accelerator card and only using the Amiga host as a power supply, keyboard and mouse...
Jens imagined FPGA replacements for ECS chipset as Clone-A concept, which lives on partially as his flickerfixer products. One could make such things do more than just the ECS chipset with flickerfixed output, add new screenmodes etc.
The emulator idea sounds interesting. There will be a lot of software to go with though.
